Communication system with fast control traffic

ABSTRACT

A method and system for conducting rapid control traffic in a time division multiple access (TDMA) communication system comprises a base station communicating with a plurality of user stations in assigned time slots of a time frame. For bearer traffic, time slots are assigned to particular user stations for an extended duration. In unassigned time slots, the base station transmits a general polling message indicating availability of the time slot. A user station desiring to hand off communication from one base station to another uses multiple available time slots at the target base station for exchanging control traffic messages with the target base station. The next available time slot is indicated by a slot pointer in the header of each general polling message to facilitate rapid exchange of control traffic messages. During handover, the user station may establish a new link with the target base station before relinquishing the existing communication link with the old base station.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. application Ser.No. 09/795,005, filed on Feb. 26, 2001, abandoned, which is acontinuation-in-part of U.S. application Ser. No. 09/407,008, filed onSep. 28, 1999, U.S. Pat. No. 7,092,372, which is a continuation of U.S.application Ser. No. 09/122,565, filed on Jul. 24, 1998, U.S. Pat. No.6,301,242, which is a continuation of U.S. application Ser. No.08/668,483, filed on Jun. 21, 1996, U.S. Pat. No. 6,005,856, which is acontinuation-in-part of application Ser. No. 08/284,053, filed Aug. 1,1994, U.S. Pat. No. 6,088,590, which is a continuation-in-part of U.S.application Ser. No. 08/215,306, filed Mar. 21, 1994, abandoned, whichis a continuation-in-part of U.S. application Ser. No. 08/146,496, filedNov. 1, 1993, abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The field of the present invention relates to wireless communicationand, more particularly, to communication protocols for control trafficin a wireless communication system.

2. Description of Related Art

A mobile communication system may generally comprise a set of “userstations”, typically mobile and the endpoints of a communication path,and a set of “base stations”, typically stationary and theintermediaries by which a communication path to a user station may beestablished or maintained. A group of base stations may be connected toa base station controller, or a cluster controller, which can in turn beconnected to a local public telephone network through, for example, amobile switching center.

It is generally desirable in a mobile communication system to achievethe greatest possible user traffic capacity at a base station, so thatfewer base stations need to be deployed in order to serve user demands.One technique used to allow a base station to communicate with multipleuser stations is use of time division multiple access (TDMA). In aparticular TDMA system, for example, a time frame is divided into aplurality of smaller time units, or time slots, and transmissions fromthe base station and from the user stations are separated in time so asto avoid collisions. In addition to separating transmissions in time,transmissions may also be distinguished by using different assignedfrequencies, thereby resulting in a frequency division multiple access(FDMA) system. Furthermore, transmissions may be encoded using spreadspectrum techniques, and different cells in a mobile communicationsystem may be assigned different spread spectrum codes, therebydifferentiating transmissions through code division multiple access(CDMA).

Generally, in order to carry out communication between a base stationand a user station, a communication link must first be established.Establishment of the communication link can be difficult in aspread-spectrum communication system, due to the length of timetypically required to synchronize the transmitter and the receiver.Establishment of the communication link and/or handing off can be moredifficult in a TDMA system in which spread spectrum is used, due to theamount of time usually necessary to synchronize the transmitter andreceiver, especially where the amount of time available forsynchronization within a user station's time slot is relatively brief.

Within a mobile communication system, a protocol generally defines howcommunication is to be initially established between a base station anda user station. The protocol may further define when and how a handoffmay be conducted as a user station leaves the service area or “cell” ofone base station and enters the service area of another base station.Messages exchanged between a base station and user station for thepurposes of establishing or maintaining a connection, or for handing offcommunication, generally can be referred to as control traffic orsignaling traffic. Messages carrying data to be conveyed between theendpoints of a call are generally referred to as bearer trafficmessages.

Initial communication between a user station and a base station can beestablished either when the user station seeks to initiate communicationwith a base station (for example, attempting to initiate a telephonecall), or when the base station attempts to complete a call to the userstation (for example, where the user station is paged). In manyconventional mobile communication systems, a dedicated control channelis used to assist mobile stations in establishing communication.According to this technique, the mobile station first communicates overthe control channel when establishing communication. The base stationthen assigns to the mobile station a “permanent” communication channelfor exchanging bearer traffic messages.

In at least one mobile communication system, however, a user station canestablish initial communication using the same channel used fortransmitting bearer traffic. For example, a system in which a userstation can establish communication by exchanging control trafficmessages in a particular communication channel (e.g., a time slot of atime frame), and thereafter use the same channel (time slot) for bearertraffic, is described in U.S. patent application Ser. No. 08/284,053filed Aug. 1, 1994, which is assigned to the assignee of the presentinvention, and hereby incorporated by reference as if set forth fullyherein.

The exchange of control traffic messages may also occur during a handoffof a user station from one base station to another, usually as the userstation moves between service areas. Typically, in the large majority ofconventional mobile communication systems, handoffs are carried outunder the direction of the base station and/or a mobility control centerconnected to the base station. When a communication link starts to breakdown, the base station requests a transfer of an ongoing call to anearby base station, which becomes the target for handoff. The targetbase station may be selected according to criteria developed at the basestation, the user station, or both. A control channel (which may be thesame dedicated control channel as used for establishing communication,where provided) may be used for the purpose of assisting the mobilestation with the handoff.

In some mobile communication systems, the user station plays a largerrole in handoff. An example of such a system is generally described inU.S. patent application Ser. No. 08/284,053, previously incorporatedherein by reference. In at least one embodiment disclosed therein, theuser station not only determines when to hand off, but also takes stepsto initiate a hand off from its current base station to a different basestation.

It is generally desirable in mobile communication systems to allow therapid establishment of communication links between mobile stations andbase stations, and rapid handoff between base stations, without errorsand without inadvertently dropping the call or losing a communicationlink. This type of capability would tend to imply the need for devotingpotentially significant resources (i.e., communication channels andprocessing speed and power) to handle link establishment and handoff.Because the communication environment can be unstable and multiple usersmay need to be serviced at the same time, a mobile communication systemis preferably capable of handling multiple service requests for linkestablishment or handoff, and doing so quickly and without errors ordropped calls.

At the same time, resources available for handling control trafficmessages are usually limited, sometimes severely so, in part becausecontrol traffic resources generally must compete against bearer trafficresources. Thus, resources dedicated to control traffic reduce theoverall resources available for handling data or bearer traffic, andvice versa. By setting aside resources (such as a dedicated controlchannel or multiple such channels) for servicing control trafficdemands, the base station's user capacity can be adversely impacted. Asa result, a greater number of base stations may need to be deployed toservice a given number of expected users.

It would therefore be advantageous to provide a communication systemhaving a rapid and reliable means for establishing a communication linkbetween a base station and a user station. It would further beadvantageous to provide a communication protocol enabling rapid handoffsand control traffic functions, and which is particularly suited to usein a time division multiple access environment. It would further beadvantageous to provide a communication protocol having a fast handoffand control traffic capability well suited to the demands of spreadspectrum communication.

SUMMARY OF THE INVENTION

In one aspect of the present invention, a method and system for handingoff communication between base stations in a mobile communication systemis provided. In a preferred embodiment of the invention, a mobilestation communicates with a base station using a time division multipleaccess (TDMA) and/or time division duplex (TDD) technique. In such anembodiment, a continuous sequence of time frames is generated, with eachtime frame comprising a plurality of time slots. The base station cancommunicate with a plurality of user stations (some or all of which maybe mobile stations), one in each time slot. A mobile station desiring tohand off exchanges a plurality of control traffic messages with a secondbase station to establish communication in a different time slot withthe second base station. The mobile station then releases thecommunication channel with the first base station and requests, throughthe second base station, the transfer of the call to the second basestation.

In a preferred embodiment of the present invention, a mobile stationtransmits and/or receives a plurality of control traffic messages inmultiple time slots of one or more time frames with the second (target)base station while in the process of handing off communication to thetarget base station, or performing other control traffic signaling. Thesecond base station provides an indication to the mobile station of thenext available time slot for control traffic, and, if desired, cantemporarily assign additional time slots to the mobile station duringhandoff, or other control traffic signaling.

In another aspect of the present invention, a method and system forestablishing communication and handing off communication in a TDMAand/or TDD communication is provided. In one embodiment, the basestation transmits a general poll message in each available time slot toindicate availability of the time slot. To establish communication in anavailable time slot, a user station responds to the general poll messagewith a general poll response. The base station then follows with aspecific poll message. The user station responds with a specific pollresponse. Normal traffic communication may thereafter be conducted overan established communication link. During normal traffic communication,in one embodiment, each user station transmits information to the basestation during an initial portion of an assigned time slot, and eachuser station receives information from the base station during a latterportion of the same assigned time slot.

Handover between base stations may be carried out by establishing a newcommunication link with a new base station, while maintaining an oldcommunication link with an original base station until the newcommunication link is fully established. The new communication link maybe established in the same manner as the original link—that is, by usingthe same handshaking technique involving a general poll, generalresponse, specific poll, and specific response messages.

In another aspect of the invention, a slot pointer information elementwithin a general polling message provides an indication of the locationof the next available time slot for communication. The slot pointer maybe a numerical value relative to the current time slot. As part of aspecific polling message, the slot pointer information element providesan assignment of the time slot channel to be used for futurecommunication by the user station presently in the process ofestablishing communication. The slot pointer may be used to performrapid handover by allowing the use of multiple time slots within a timeframe for control traffic.

In another embodiment, virtual time slots are defined as part of thetiming structure. As used herein, a virtual time slot is generally atime slot assigned to the same user station with two transmissionintervals non-adjacent in time. For example, a virtual time slot may bea time slot in which a forward link transmission and a reverse linktransmission for a particular user station are separated bytransmissions to or from one or more other user stations. In a preferredsystem in which each physical time slot has a user transmission intervaland a base transmission interval, a user station may therefore transmita user message to the base station during a user transmission intervalof a first physical time slot, and receive a base message from the basestation during a base transmission interval of a second, subsequentphysical time slot. In a particular embodiment, a virtual slot field inthe header of the general polling message indicates whether or notvirtual time slots are provided, thereby enabling operation in either oftwo modes, one using virtual time slots and the other not using virtualtime slots.

A method and system for establishing and maintaining spread spectrumcommunication is disclosed with respect to a preferred embodimentwherein data symbols are encoded using an M-ary direct sequence spreadspectrum communication technique. Further variations and details of theabove embodiments are also described herein and/or depicted in theaccompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic representation of a cellular communicationsystem.

FIG. 1A is a diagram of an arrangement of cells in a wirelesscommunication system showing an exemplary code and frequency reusepattern.

FIG. 2 is a diagram of one embodiment of a communication system.

FIG. 2A is a block diagram of another embodiment of a communicationsystem, using a GSM-based network interconnection.

FIG. 3 is a diagram of a time frame divided into time slots.

FIG. 4 is a diagram illustrating a protocol for establishing acommunication link between a base station and a user station.

FIG. 4A is a message flow diagram corresponding to FIG. 4.

FIG. 5A is a diagram of a preferred time slot structure.

FIGS. 5B and 5C are diagrams of a base station transmit data time framestructure and a user station transmit data time frame structure,respectively.

FIG. 6 is a diagram of a time frame structure in accordance with anotherembodiment of the invention showing a time frame divided into virtualtime slots.

FIGS. 7A-7C are diagrams of polling message formats.

FIGS. 8A and 8B are diagrams of message header formats.

FIG. 9 is a message flow diagram illustrating call origination from auser station.

FIG. 10 is a message flow diagram illustrating call termination at theuser station.

FIGS. 11A-11C are message flow diagrams illustrating a handover of amobile call between two base stations within a cluster.

FIGS. 12A and 12B are message flow diagrams illustrating a handover of amobile call between two base stations located in different clusters.

FIGS. 13A and 13B are diagrams of a base station data packet and a userstation data packet, respectively.

FIGS. 14A and 14B are timing diagrams showing a time frame and time slotstructure in a linear representation and loop representation,respectively.

FIG. 15 is a diagram of a series of consecutive time frames showingutilization of a particular time slot over a sequence of time frames.

FIGS. 16A and 16B are timing diagrams of mobile station transmissionsand base station transmissions, respectively, within a particularpolling loop of the type shown in FIG. 14B, wherein symmetric time slotsare used.

FIGS. 17A and 17B are timing diagrams of mobile station transmissionsand base station transmissions, respectively, within a particularpolling loop of the type shown in FIG. 14B, wherein asymmetric timeslots are used.

FIGS. 18A and 18B are timing diagrams showing multiple time slotsutilized for carrying out control traffic operations.

FIG. 19 is a block diagram of a communication system illustratinginter-cluster and intra-cluster handoffs.

FIG. 20 is a block diagram of a transmitter and a receiver in a spreadspectrum communication system.

FIG. 21 is a diagram illustrating a preferred system protocolarchitecture.

FIG. 22 is a call flow diagram of a call release initiated by a userstation.

FIG. 23 is a call flow diagram of a call release initiated by thenetwork.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a diagram of a pattern of cells for a multiple-access wirelesscommunication system 101. The wireless communication system 101 of FIG.1 includes a plurality of cells 103, each with a base station 104,typically located at the center of the cell 103. A plurality of userstations 102, some or all of which may be mobile, communicate with thebase stations 104 to place and receive calls. Each station (both thebase stations 104 and the user stations 102) generally comprises areceiver and a transmitter.

A control station 105 may also be provided (comprising a receiver and atransmitter) to manage the resources of the system 101. The controlstation 105 (which may comprise a “base station controller” as describedlater herein) may assign the base station 104 and user stations 102 ineach cell 103 a spread-spectrum code or a set of spread spectrum codesfor modulating radio signal communication in that cell 103.(Alternatively, a spread spectrum code or set of spread spectrum codesmay be pre-assigned to a cell 103.) The resulting spread spectrumsignals are generally spread across a bandwidth exceeding the bandwidthnecessary to transmit the data, hence referred to by the term “spreadspectrum.” Accordingly, radio signals used in a cell 103 are preferablyspread across a bandwidth sufficiently wide that both base station 104and user stations 102 in an adjacent cell 103 can distinguishcommunication which originates in the first cell 103 from communicationwhich originates in the adjacent cell 106.

FIG. 2 is a block diagram of a communication system architectureutilized in a preferred embodiment of the present invention. The FIG. 2communication system comprises a plurality of base stations 104 forcommunicating with a plurality of user stations 102. The base stations104 and user stations 102 may operate in a personal communicationssystem (PCS), such au may be authorized under rules prescribed by theFederal Communications Commission (FCC).

Each base station 104 may be coupled to a base station controller 105 byany of a variety of communication paths 109. The communication paths 109may each comprise one or more communication links 118. Eachcommunication link 118 may include a coaxial cable, a fiber optic cable,a digital radio link, or a telephone line.

Each base station controller 105 may also be connected to one or morecommunication networks 126, such as a public switched telephone network(PSTN) or personal communication system switching center (PCSC). Eachbase station controller 105 is connected to a communication network 126by means of one or more communication paths 108, each of which mayinclude a coaxial cable, a fiber optic cable, a digital radio link, or atelephone line.

The FIG. 2 communication system also may include one or more“intelligent” base stations 107 which connect directly to acommunication network 126 without interfacing through a base stationcontroller 105. The intelligent base stations 107 may therefore bypassthe base station controller 105 for local handoffs and switching of userstations 102, and instead perform these functions directly over thenetwork 126.

In operation each base station 104 formats and sends digital informationto its respective base station controller 105 (or directly to thenetwork 126 in the case of an intelligent base station 107). The basestation controllers 105 receive inputs from multiple base stations 104,assist handoffs between base stations 104, and convert and formatchannel information and signaling information for delivery to thenetwork 126. The base station controllers 105 may also manage a localcache visitor location register (VLR) database, and may support basicoperation, administration and management functions such as billing,monitoring and testing. Each base station controller 105, under controlof the network 126, may manage local registration and verification ofits associated base stations 104 and may provide updates to the network126 regarding the status of the base stations 104.

The network 126 connects to the base station controllers 105 for calldelivery and outgoing calls. Intelligent base stations 107 may use ISDNmessaging for registration, call delivery and handoff over a publictelephone switch. The intelligent base station 107 may have all thegeneral capabilities of a base station 104 but further incorporate abasic rate. IDN (BRI) card, additional intelligence and local vocoding.

The communication system may also be based on a GSM networkinterconnection. FIG. 2A is a diagram of a communication systemarchitecture showing such an interconnection. In the communicationsystem shown in FIG. 2A, the base stations 104 may connect to a GSMmobile switching center 112 through a GSM “A” interface. The “A”interface may be incorporated in base station controllers 105 and inintelligent base stations 107. Features and functionality of GSM may bepassed to and from the base stations 104 over the “A” interface in amanner that is transparent to the end user (i.e., user stations 102).The GSM mobile switching center 112 may connect to a PSTN or to othernetworks, as indicated in FIG. 2A.

The system may also interconnect to cable television distributionnetworks. In such a system, the base stations 104 may be miniaturized sothat they can be installed inside standard cable TV amplifier boxes.Interfacing may be carried out using analog remote antenna systems anddigital transport mechanisms. For example, T1 and fractional T1 (“FT1”)digital multiplexer outputs from the cable TV network may be used forinterfacing, and basic rate (BRI) ISDN links may be used to transportdigital channels.

FIG. 1A is a diagram of a preferred cellular environment in which theinvention may operate. According to FIG. 1A, a geographical region 201is divided into a plurality of cells 103. Associated with each call 103is an assigned frequency and an assigned spread spectrum code.Preferably, three different frequencies (or frequency groups) F1, F2 andF3 are assigned in such a manner that no two adjacent cells have thesame assigned frequency (or frequency group) F1, F2 or F3, therebyminimizing RF interference between adjacent cells. The frequencies maybe assigned on a “permanent” basis, or else dynamically through thenetwork.

To further reduce the possibility of intercell RF interference,different near-orthogonal spread spectrum codes C1 through C7 areassigned as shown in a repeating pattern overlapping the frequency reusepattern. Although a repeating pattern of seven spread spectrum codes C1through C7 is preferred, a pattern involving other numbers of spreadspectrum codes may be suitable depending upon the particularapplication. As with frequencies used in the cells 103, spread spectrumcodes may be assigned on a “permanent” basis or else dynamically throughthe network. Further information regarding a suitable cellularenvironment for operation of the invention may be found in U.S. Pat. No.5,402,413, assigned to the assignee of the present invention, and herebyincorporated by reference as if fully set forth herein.

The use of spread spectrum for carrier modulation permits a frequencyreuse factor of N=3 for allocating different carrier frequencies F1, F2and F3 to adjacent cells 103. Interference between cells 103 using thesame carrier frequency F1, F2 or F3 is reduced by the propagation lossdue to the distance separating the cells 103 (i.e., any two cells 103using the same frequency F1, F2 or F3 are separated by at least oneintervening cell 103, as shown in FIG. 1A), and also by the spreadspectrum processing gain obtained by the use of near-orthogonalspreading codes.

Further details regarding an exemplary cellular pattern are describedin, e.g., U.S. Pat. No. 5,402,413 referred to above.

A preferred embodiment of the invention achieves multiple accesscommunication by using a time frame divided into multiple time slots,i.e., time division multiple access (TDMA). FIG. 3 is a diagram showinga timing structure for a particular TDMA system. According to the timingstructure of FIG. 3, communication over time is broken into a continuousseries of time frames 301. A single complete time frame 301 is shownalong a timeline 310 in FIG. 3; similar time frames are assumed toprecede and follow time frame 301 in a continuous pattern along thetimeline 310.

Time frame 301 is divided into a plurality of time slots 302 numberedconsecutively TS1, TS2 . . . TSN, each of which may support duplexcommunication with a user station 102. Time frame 301 may be thought ofas a “polling loop” or a time loop, as depicted in FIG. 3, whereby userstations 102 are communicated with sequentially over the time frame 301in a manner analogous to polling, each user station 102 transmitting andreceiving messages in its designated time Blot 302. In the FIG. 3embodiment, each time slot 302 comprises a user transmission interval305, wherein a user station 102 transmits a user-to-base message to thebase station 104, and a base transmission interval 306, wherein the basestation 104 transmits a base-to-user message to the user station 102.Communication in time slots 302 may be interleaved, such that userstations 102 transmit in one physical time slot 302 but receive in adifferent physical time slot 302.

In an exemplary TDMA communication system, time frames 301 are each inthe neighborhood of 20 milliseconds in duration, and each time frame 301comprises sixteen time slots 302 or, alternatively, eight time slots 302to support extended range through increased guard times.

In some embodiments, a user station 102 may communicate in more than onetime slot 302 in each time frame 301, so as to support an increased datarate. Similarly, in some embodiments, a user station 102 mayperiodically skip time frames 301 and communicate in some subset of alltime frames 301 (e.g., every other time frame 301, or every fourth timeframe 301), so as to support a reduced data rate where a full speedcommunication link is not necessary. Further information about anexemplary TDMA system supporting variable data rates as described abovemay be found in copending U.S. patent application Ser. No. 08/284,053filed Aug. 1, 1994, previously incorporated herein by reference.

FIG. 6 is a diagram of a timing structure employing virtual time slots,each of which generally comprises a duplex pair (i.e., one forward linkand one reverse link).

In FIG. 6, similar to FIG. 3, communication over time is broken into acontinuous series of time frames 601. A single complete time frame 601is shown along a timeline 610 in FIG. 6; similar time frames are assumedto precede and follow time frame 601 in a continuous pattern along thetimeline 610.

Time frame 601 is divided into a plurality of physical time slots 602numbered consecutively TS1′, TS2′ . . . TSN′. Each physical time slot602 comprises a user transmission interval 605 wherein a user station102 transmits a user-to-base message to the base station 104, and a basetransmission interval 606 wherein the base station 104 transmits abase-to-user message to a user station 102, which could be a differentuser station 102 than transmitted to the base station 104 in the samephysical time slot 602. Using virtual time slots, communication inphysical time slots 602 may be interleaved, such that a user station 102transmits in one physical time slot 602 but receives in a differentphysical time slot 602. The user transmission interval 605 and basetransmission interval 606 which define the forward link and reverse linktransmissions for a given user station 102 (and which are generallylocated in different physical time slots 602, as depicted in FIG. 6) arecollectively referred to as a “virtual time slot.”

An exemplary virtual time slot 618 is shown in FIG. 6, associated with aparticular user station 102 (e.g., user station MS2). The virtual timeslot 618 comprises two message transmission intervals, one in each oftwo physical time slots 602 a and 602 b. Virtual time slot 618 has auser transmission interval 605 a in the first physical time slot 602 a,and a base transmission interval 606 b in the second physical time slot602 b. Between the user transmission interval 605 a and the basetransmission interval 606 b of the virtual time slot 618, the basestation 104 transmits in a base transmission interval 606 a of the firstphysical time slot 602 a (e.g., to a second user station 102, such asuser station MS1), and another user station 102 (e.g., a third userStation 102, such as user station MS3) transmits in a user transmissioninterval 605 b to the base station 104. In this manner, transmissions toand from the base station 104 are interleaved.

Time frame 601 may be thought of as a “polling loop” or a time loop,similar to time frame 301 of the FIG. 3 embodiment, whereby userstations 102 are communicated with sequentially over the time frame 601in a manner analogous to polling, each user station 102 transmitting andreceiving messages in its designated virtual time slot 618. The virtualtime slots 618 of FIG. 6, however, are not necessarily identical to thephysical time slots 602. An advantage of the FIG. 6 timing structure isthat it may allow extended time for the base station 104 to processchannel characterization data as received from the user station 102.

In an exemplary TDMA communication system, time frames 601 are each 20milliseconds in duration, and each time frame 601 comprises sixteen timeslots 602 or, alternatively, eight time slots 602 to support extendedrange through increased guard times.

Further details regarding time frame structures (including virtual timeslots) may be found in copending U.S. patent application Ser. No.08/668,483 filed Jun. 21, 1996, hereby incorporated by reference as ifset forth fully herein.

In some embodiments, a user station 102 may communicate in more than onevirtual time slot 618 in each time frame 601, so as to support anincreased data rate. Similarly, in some embodiments, a user station 102may periodically skip time frames 601 and communicate in some subset ofall time frames 601 (e.g., every other time frame 601, or every fourthtime frame 601), so as to support a reduced data rate where a full speedcommunication link is not necessary.

Communication between a user station 102 and a base station 104 isestablished in one embodiment by a response from a user station 102 to ageneral polling message sent from the base station 104 during anavailable time slot 302. This process is described in more detail withreference to FIG. 4, which illustrates a protocol for establishment of aspread spectrum communication link in, e.g., the FIG. 3 communicationsystem. A communication link may be established in an analogous mannerfor the FIG. 6 embodiment.

In the FIG. 4 protocol, a general poll message 401 is transmitted by thebase station 104 in some or all of the time slots 302 which areavailable for communication. A user station 102 may monitortransmissions from a base station 104 and ascertain available time slots302 by receiving general poll messages 401 in those time slots 302.

A user station 102 may “acquire” a base station 104 by a sequence ofhandshaking steps. At a general poll step 407, the base station 104transmits its general poll message 401 during an unoccupied time slot302. The user station 102 receives the general poll message 401 and, ifit was received without error, transmits a general poll response 404 tothe base station 104 in the same time slot 302 of the following timeframe 301 (or in a different time slot, as explained hereafter). Thegeneral poll message 401 preferably comprises a field for a base ID 408b, which may be 32 bits long (for example), and which may be stored orotherwise recorded by the user station 102. Similarly, the general pollresponse 404 preferably comprises a field for a user ID 409, which maybe 32 bits long (for example), and which may be stored or otherwiserecorded by the base station 104.

Upon receiving a general poll response 404, at a specific poll step 410the base station 104 transmits a specific poll message 402 comprising(among other things) the user ID 409 which had been previously receivedby the base station 104 as part of the general poll response 404. Theuser station 102 receives the specific poll message 402 and, if it wasreceived without error and with the same user ID 409, transmits itsspecific poll response 405 to the base station 104 in the same time slot302 of the following time frame 301 (or in a different time slot, asexplained further herein). The specific poll response 405 comprises thesame user ID 409 as the general poll response 404.

In a particular embodiment, the specific poll response 405 may beeliminated as redundant. The user station 102 may, in such a case,follow the specific poll message 402 with a user traffic message 406.

Upon receiving a specific poll response 405 comprising a user ID 409which matches that of the general poll response 404, at alink-established step 411 the base station 104 may transmit a trafficmessage 403. At this point, the base station 104 and user station 102have established a communication link 412. The base station 104 mayconnect a call through the communication channel, and the user station102 may begin normal operation on a telephone network (e.g., the userstation 102 may receive a dial tone, dial a number, make a telephoneconnection, and perform other telephone operations). The base station104 and user station 102 may exchange traffic messages 403 and 406,until the communication link 412 is voluntarily terminated, until faultycommunication prompts the user station 102 to re-acquire the basestation 104, or until handoff of the user station 102 to another basestation 104.

FIG. 4A illustrates a similar exchange of messages in a message flowdiagram format, whereby a user station 102 establishes communicationwith a base station 104.

Should more than one user station 102 respond to the same general pollmessage 401, the base station 104 may intentionally fail to respond witha specific poll message 402. The lack of response from the base station104 signals the involved user stations 102 to back off for a calculatedtime interval before attempting to acquire the same base station 104using the general poll message 401 and general poll response 404protocol. The back-off time may be based upon the user ID 409, andtherefore each user station 102 will back off for a different length oftime to prevent future collisions, in a manner similar to that specifiedby IEEE Standard 802.3.

When an incoming telephone call is received at a base station 104 at anincoming-call step 413, the base station 104 skips the general poll,message 401 and general poll response 404 and moves directly to thespecific poll step 410. The base station 104 transmits a specific pollmessage 402 with the user ID 409 of the indicated recipient user station102 on an available time slot 302. As further described herein, eachuser station 102 listens regularly for the specific poll message 402 soas to receive the specific poll message 402 within a predetermined timeafter it is transmitted. When the specific poll message 402 is received,the user station 102 compares the user ID 409 in the message with itsown user ID, and if they match, continues with the link-established step411. The base station 104 may thereby establish a communication link 412with any user station 102 within communication range.

Further details regarding means for establishing communication(particularly spread spectrum communication) in a TDMA system may befound in copending U.S. Pat. No. 5,455,822 and in copending U.S. patentapplication Ser. No. 08/284,053 filed Aug. 1, 1994, both of which arehereby incorporated by reference as if fully set forth herein.

In a preferred embodiment, the general poll message 401 comprises a nextslot pointer (contained in a next slot pointer field 810 shown in anddescribed with respect to FIG. 8A) which indicates the next time slot302 (or virtual time slot 618) during which a general poll message 401will be transmitted by the base station 104. In such an embodiment, auser station 102 seeking to establish communication responds to thegeneral poll message 401 in the user transmission interval 305 (or 605)of the time slot 302 (or 618) indicated by the next slot pointer, andnot necessarily in the same time slot of the next time frame 301 (or601). Upon receiving a general response message 404 from the userstation 102 in the time slot indicated by the next slot pointer, thebase station 102 responds with a specific poll message 402. Should morethan one user station 102 respond to a general poll message 401, theappearance of a general poll message 401 (rather than a specific pollmessage 402) in the time slot indicated by the next slot pointer willcause each user station 102 involved to back off for a variable periodof time depending on the user station ID.

The specific poll message 402 comprises a temporary shorthand identifier(nickname) specific to the user station 102 and referred to herein as a“correlative ID.” The correlative ID appears in subsequent signalingmessages (in both directions) until the established link is dropped. Inresponse to the specific poll message 402, the user station 102 respondswith a traffic message in a time slot 302 (or 618) assigned by a nextslot pointer in the header of the specific poll message 402.

Further details of how the next slot pointer (sometimes referred tosimply au the slot pointer) is used within preferred embodiments aredescribed below, after a brief description of various time intervalswithin a time slot and basic message structures and formats. Theparticular time intervals, messages structures and formats are meant tobe illustrative and to represent various preferred embodiments fordemonstrating the workings of the invention, and are not meant to limitthe invention to any particular type of message structure or format, orany particular type of time slot structure.

FIG. 5A is a diagram of a preferred slot structure, and FIGS. 5B and 5Care diagrams of a base station transmit data frame structure and a userstation transmit date frame structure, respectively. In FIG. 5A, a timeslot 510 comprises a variable radio delay gap 505, a user stationtransmit frame 515, a base processor gap 525, a guard time 535, a basestation transmit frame 545, and a radar gap 555. Each user stationtransmit frame 515 comprises a user preamble 516, a user preamblesounding gap 519, and a user station transmit data frame 521. Similarly,each base station transmit frame 545 comprises a base preamble 547, abase preamble sounding gap 549, and a base transmit data frame 551.

FIG. 5B illustrates a preferred message structure for the base stationtransmit data frame 551. The message structure of FIG. 5B comprises abase header field 553, a base D-channel field 557, a base data field559, and a base cyclical redundancy check (CRC) field 561. In apreferred embodiment, the base header field 553 is 23 bits, the baseD-channel field 557 is 8 bits, the base data field 559 is 192 bits, andthe base CRC field 561 is 16 bits.

FIG. 5C illustrates a preferred message structure for the user stationtransmit data frame 521. The message structure of FIG. 5C comprises auser header field 523, a user D-channel field 527, a user data field529, and a user CRC field 531. In a preferred embodiment, the userheader field 523 is 17 bits, the user D-channel field 527 is 8 bits, theuser data field 529 is 192 bits, and the user CRC field 531 is 16 bits.

FIGS. 7A-7C are diagrams of preferred polling message formats. FIG. 7Ais a diagram of a general poll message format, such as may be employed,for example, with general poll message 401 of FIG. 4. As shown in FIG.7A, a general poll message 701 preferably comprises, in the followingsequence, a header field 702, a spare field 703, a zone field 704, abase station controller (BSC) ID field 705, a base ID field 706, afacility field 707, a system type field 708, a service provider field709, a slot quality field 710, a forward error correction (FEC) field711, and a frame control word (FCW) field 712. In a preferredembodiment, the header field 702 is 24 bits long, the spare field 703 is16 bits long, the zone field 704 is 40 bits long, the BSC ID field 705is 16 bits long, the base ID field 706 is 32 bits long, the facilityfield 707 is 32 bits long, the system type field 708 is 8 bits long, theservice provider field 709 is 16 bits long, the slot quality field 710is 8 bits long, the FEC field 711 is 32 bits long, and the frame controlword field 712 is 16 bits long, for a total of 240 bits.

The header field 702 identifies the message type and is described morefully with respect to FIG. 8A. The zone field 704 identifies the pagingzone of the specific base station 104. A user station 102 may move fromone base station 104 service area to another in the same zone withoutrequiring immediate re-registration. The BSC ID field 705 is a sequenceuniquely identifying the base station controller 105. The base ID field706 is a sequence uniquely identifying the base station 104. Thefacility field 707 describes the services offered by the base station104 (e.g., internet access, aggregate data capability, enhanced voice,etc.). The facility field 707 may include a sub-field indicating whatuser stations may have access to the channel (e.g., 911 calls only, oruser stations 102 with specific access codes). The system type field 708identifies the type of system associated with the base station 104. Theservice provider field 709 identifies the PCS service provider thatoperates the base station 104 (or, if more than one service provider isavailable at the base station 104, the service provider that currentlyoperates the particular time slot). The slot quality field 710 indicatesthe relative quality of the time slot in terms of interference.Generally, the lower the number, the better the slot quality. The FECfield 711 is used for forward error correction. The FCW field 712 isused for error detection, and in one embodiment comprises a sequence ofbits and/or phase shifts determined according to following algorithm:

-   -   1. Calculate remainder R1 of a seed polynomial SDP modulo-2        divided by a generator polynomial GRP;    -   2. calculate product P of x¹⁶ and content of the message 701        preceding FCW field 710;    -   3. Calculate remainder R2 of the generator polynomial GNP        modulo-2 divided by the product P derived in Step 2;    -   4. Calculate modulo-2 sum S of remainder R1 and remainder R2;        and    -   5. Calculate the ones-complement of sum S the result of which is        transmitted in the FCW field 710.        In a preferred embodiment, the seed polynomial SDP is:        x^(k)(x¹⁵+x¹⁴+x¹³+x¹²+x¹¹+x¹⁰+x⁹+x⁸+x⁷+x⁶+x⁵+x⁴+x³+x²+x¹+1)        and the generator polynomial GRP is:        x¹⁶+x¹²+x⁵+1

FIG. 7B is a diagram of a specific poll message format (such as may beemployed, for example, with specific poll message 402 of FIG. 4). Asshown in FIG. 7B, a specific poll message 720 preferably comprises, inthe following sequence, a header field 721, a correlative ID field 722,a cause field 723, a personal identifier (PID) field 724, anover-the-air (OTA) map type field 725, an OTA map field 726, a sparefield 727, a slot quality field 728, a forward error correction field729, and an FCW field 730. In a preferred embodiment, the header field721 is 24 bits long, the correlative ID field 722 is 8 bits long, thecause field 723 is 8 bits long, the PID field 724 is 72 bits long, theOTA map type field 725 is 8 bits long, the OTA map field 726 is 32 bitslong, the spare field 727 is 32 bits long, the slot quality field 728 is8 bits long, the FEC field 729 is 32 bits long, and the FCW field 729 is16 bits long, for a total of 240 bits.

The header field 721, slot quality field 728, FEC field 729, and FCWfield 730 are similar to the analogous fields described for FIG. 7A. Thecorrelative ID field 722 is used to temporarily identify one or morechannels (i.e., time slots) as being allocated to a specific userstation 102. A correlative ID number is assigned for the duration of acall connection and is released for reuse by another user station 102 atthe termination of a connection; the correlative ID number may also bechanged during a connection. A specific correlative ID number may bereserved by the base station 104 for broadcast use. The cause field 723indicates the cause of an error occurring during execution of a previoussignaling traffic operation for the particular user station 102.Interpretation of the cause field 723 message may therefore depend uponthe type of signal traffic involved. Possible cause messages include,for example, those indicating that the user station 102 is unregisteredor will not be accepted for registration, or that the call has not beenconnected or cannot be completed. The PID field 724 comprises a personalidentification number which uniquely identifies the subscriber (e.g.,user station 102). The OTA map type field 725 defines the type of map(e.g. superframe, subframe, etc., as defined later herein) that followsin the OTA map field 726. The OTA map field 726 describes the mapping oftime slots relative to a particular user station 102. The format of theOTA map field 726 depends on the map type.

FIG. 7C is a diagram of a poll response message format (ouch as may beemployed, for example, with general poll response 404 or specific pollresponse 405 of FIG. 4). As shown in FIG. 7C, a poll response message740 preferably comprises, in the following sequence, a header field 741,a first spare field 742, a PID field 743, a service provider field 744,a class field 745, a user capabilities field 746, a second spare field747, an FEC field 748, and an FCW field 749. In a preferred embodiment,the header field 741 is 17 bits long, the first spare field 742 is 16bits long, the PID field 743 is 72 bits long, the service provider field744 is 16 bits long, the class field 745 is 16 bits long, the usercapabilities field 746 is 16 bits long, the second spare field 747 is 32bits long, the FEC field 748 is 32 bits long, and the FCW field 749 is16 bits long, for a total of 233 bits.

The header field 741 identifies the message type and is more fullydescribed in FIG. 8B. The PID field 743, FEC field 748, and FCW field746 are similar to the PID field 724, FEC field 729, and FCW field 730,respectively, described with respect to FIG. 7B. The service providerfield 744 identifies the PCS service provider that the user station 102wishes to use. The class field 745 specifies some of the operationalparameters being used by the particular user station 102. The classfield 745 may comprise a class type sub-field and a class informationsub-field. The class type sub-field indicates the user station classtype (e.g., DCS1900 class type, or IS-41 class type, etc.), and may alsoprovide an indication of the power level capability of the user station102. The class information sub-field provides operational informationincluding, for example, revision level, available encryption algorithms,short message capability, ellipsis notation and phase-2 error handlingcapability, power class, continuous/discontinuous transmission,bandwidth (e.g., 20 MHz or 25 MHz), and nominal power levels. The classtype sub-field may, for a GSM-oriented system, indicate the power levelcapability of the user station 102. The user capabilities field 746identifies the features present in the user station 102 (e.g., whetherthe user station 102 can receive a fax or data connection, whether theuser station 102 is capable of ciphering, etc.).

FIGS. 8A and 8B are diagrams of preferred polling message headerformats. FIG. 8A is a diagram of a polling message header format for abase polling message (such as general poll message 401 or specific pollmessage 402 of FIG. 4). The polling message header 801 comprises abase/mobile indicator (B/M) flag 802, an extended protocol (E) flag 803,a packet type field 804, a power adjustment (PWR) field 805, a symmetryfield 806, a D-channel suppression (DCS) flag 807, a virtual slot (VS)flag 808, a slot or channel utilization (CU) field 809, a slot pointerfield 810, a error check and correct (ARQ) field 811, and a header framecontrol word (HCF) field 812. In a preferred embodiment, the B/Mindicator flag 802, E flag 803, PWR field 805, DCS flag 807, and the VSflag 808 are each 1 bit long, the packet type field 804 and symmetryfield are each 2 bits long, the CU field 609 and ARQ field are each 3bits long, and the slot pointer field 810 and header HCF field 812 areeach 4 bits long, for a total of 23 bits. A twenty-fourth bit of theheader 801 is used for the purpose of assisting establishment of the RFlink.

The B/M indicator flag 802 indicates whether the originator of themessage is a user station 102 or the base station 104. The E flag 803 isused to indicate whether or not an extended protocol is in use. Thepacket type field 804 specifies which of four packet types is beingused, according to Table 8-1A below.

TABLE 8-1A Packet Field Packet Type 00 Normal traffic 01 Specific poll10 Control (signaling) traffic 11 General poll, or general response

The packet type field 804 also provides an indication of the usage ofthe D-field 557, according to Table 8-1B below.

TABLE 8-1B Packet Field D-Field Usage 00 D-Channel 01 Correlative ID 10Correlative ID 11 Reserved

The PWR field 805 is a serialized bit stream from the base station 104to the user station 102 allowing control of the power level of the userstation 102 transmitter. As each base-to-user message is received at theuser station 102, the PWR bit from the last message is analyzed alongwith the current PWR bit to determine if the power level of the userstation 102 transmitter should be raised, lowered or remain unchanged.Power control action therefore requires that at least two consecutivebase-to-user messages be received by the user station 102 before anyaction in taken. The action taken is dictated according to Table 8-2appearing below.

TABLE 8-2 Last Bit Current Bit Action 0 0 Decrease transmitter power 1 1Increase transmitter power 0 1 Leave power unchanged 1 0 Leave powerunchanged missing any Leave power unchanged any missing Leave powerunchangedThe amount of power increase or decrease carried out in response toreceiving commands in the PWR field 805 may be a fixed or presetamount—e.g., 1 dB for each time frame 301 (or more frequently if theuser station 102 is transmitting in multiple time slots 302 per timeframe 301). Using only a single bit for the PWR field 805 saves space inthe header 553 of the base-to-user message. The quality metricsgenerally provide sufficient feedback to allow small power adjustmentsteps over time, but not sufficient feedback to have confidence inmaking substantial power adjustment steps. However, because user stationtransmissions are separated by time within the general geographic regionof a particular base station 104, strict power control of the userstations 102 is not required to avoid intracell or intercellinterference as it typically is with CDMA systems not employing timedivision techniques.

The symmetry field 806 is used by the base station 104 to grantbandwidth to the user station 102. The bandwidth grant applies to thenext time slot 302 (or 618) in the channel. The symmetry field 806contents may be interpreted according to Table 8-3 below.

TABLE 8-3 Symmetry Bits Meaning 00 Symmetric bandwidth grant. Eachdirection has been granted one half of the bandwidth. 01 The maximumbandwidth has been granted to the user station 102, and the minimumbandwidth has been granted to the base station 104. 10 The maximumbandwidth has been granted to the base station 104, and the minimumbandwidth has been granted to the user station 102. 11 Broadcast mode.The entire bandwidth has been granted to the base station 104. There isno user station 102 packet.

The DCS flag 807 indicates the usage of the D-channel for the currentmessage. The DCS flag 807 is set to one value to indicate that theD-channel is disabled to reserve it for use by the application using thebearer channel (B-channel), and is set to another value to indicate thatthe D-channel is enabled for other usage. The VS flag 808 indicateswhether the base station 104 is using a virtual slot mode. If thevirtual slot mode is active (e.g., the time slot structure of FIG. 6 isused), then all user station 102 transmissions occur one time slotearlier than if the VS mode is inactive.

The CU field 809 indicates the relative slot utilization for the basestation 104. In a preferred embodiment, the CU field contents aredefined according to Table 8-4 below,

TABLE 8-4 CU Field Contents Utilization 000 No channels available: Findanother base station 001 One channel available: 911 calls only 010 Twochannels available: 911 calls or handover only 011 Few channelsavailable: Class control is in effect for registrations and originations100 Nearly full: Access Unrestricted 101 Moderately full: AccessUnrestricted 110 Partially full: Access Unrestricted 111 All slotsavailable: Access UnrestrictedWhere class control is in effect for registrations and calloriginations, access leveling and load leveling classes may beidentified in the facility field 707 of the general poll message (seeFIG. 7A).

The slot pointer field 810 contains an index which identifies the nexttime slot to be used in the current base/user packet exchange. The userstation 102 transmits in the time slot indicated by the slot pointer tocontinue the exchange. In a particular embodiment, the contents of theslot pointer field Bio may take on any of sixteen different values(e.g., binary 0 to 15), with each value indicating a different relativenumber of time slots from the present time slot in which the userstation 102 is to transmit. For example, a value of zero means that theuser station 102 is to transmit in the same slot (in the next frame ifat a regular bandwidth rate, or several frames in the future it using asub-frame rate). A value of one means that the user station 102 is totransmit in the next time slot of the present time frame. A value of twomeans that the user station 102 is to transmit in the time slot twoplaces ahead in the present time frame, and so on. Examples of operationusing slot pointers are described further below.

The ARQ field 811 allows the receiving entity (either base station 104or user station 102) to correct a message error. The ARQ field 811comprises three subfields of one bit each: (1) an “ARQ required”sub-field that indicates whether or not ARQ is required for the messagesent; (2) an “ACK” sub-field indicating whether or not the sender of themessage received correctly the last message sent; and (3) a “messagenumber” sub-field, which indicates the message number (zero or one) ofthe current message. The ACK sub-field and message number sub-field arealways used regardless of whether the ARQ required bit is set.

If ARQ is required (as determined by the value of the ARQ required bit),then the receiving entity performs the following steps:

-   -   (1) Compares the message number sub-field of the received        message with the message-number sub-field of the previously        received message; if they are the same, the new message is        ignored.    -   (2) Checks the ACK sub-field of the received message. If the        value is NAK (indicating that the sender of the message did not        receive the last message correctly), then the receiving entity        resends the old data message; otherwise, it sends a new data        message.    -   (3) Complements the message number sub-field bit each time a new        data message is sent.    -   (4) If a message is received with a FCW error (as explained with        respect to FIG. 7A), or did not receive a message at all, then        the receiving entity sends its data message with the ACK        sub-field set to NAK.

The header HCF field 812 is used for a cyclic redundancy checkcalculated over the preceding bits of the message header.

FIG. 8B is a diagram of a polling message header format for a pollresponse message (such as general poll response 404 or specific pollresponse 405 of FIG. 4). The polling response header 820 comprises abase/mobile indicator (B/M) flag 821, an extended protocol (E) flag 822,a packet type field 823, a PWR field 824, a symmetry field 825, a DCSflag 826, a spare field 827, an ARQ field 828, and a header framecontrol word (HCF) field 829. In a preferred embodiment, the B/Mindicator flag 821, E flag 822, and DCS flag 826 are each 1 bit long,the packet type field 823, symmetry field 825, and spare field 827 areeach 2 bits long, the ARQ field 828 is 3 bits long, and the HCF field829 is 4 bits long, for a total of 17 bits.

The B/M indicator flag 821, E flag 822, packet type field 823, PWR field824, DCS flag 826, ARQ field 828 and HCF field 829 are used for the samepurposes as their counterpart fields in the base station header-shown inFIG. 8A. The contents of the symmetry field 825 in the user station 102header may be interpreted according to Table 8-5 below.

TABLE 8-5 Symmetry Field Meaning 00 Symmetric bandwidth is requested forthe next time slot 01 Maximum bandwidth is requested for the next timeslot 10, 11 (Not presently used)

In one embodiment in accordance with the header formats of FIGS. 8A and8B, the message headers shown in Table 8-6 correspond to the messagetypes shown (where “1” and “0” are bit values, and “X” is a bit valuethat is irrelevant or depends upon the application and/or systemstatus).

TABLE 8-6 Message Type Header Contents BS General Poll 1X11 XXXX XXXXXXXX XXXX XXX BS Specific Poll 1X01 XXXX XXXX XXXX XXXX XXX BS ControlTraffic 1X10 XXXX XXXX XXXX XXXX XXX BS Traffic Message 1X00 XXXX XXXXXXXX XXXX XXX MS General Response 0X11 XXXX XXXX XXXX X MS SpecificResponse 0X01 XXXX XXXX XXXX X MS Control Traffic 0X10 XXXX XXXX XXXX XMS Traffic Message 0X00 XXXX XXXX XXXX X

FIG. 13A is a diagram of a base station information packet showing inoctet format fields generally depicted in FIGS. 5B and 8A. FIG. 13B is adiagram of a user station information packet showing in octet formatfields generally depicted in FIGS. 5C and 8B.

Data may be transmitted between the base station 104 and user stations102 using an M-ary spread spectrum technique. Suitable M-ary spreadspectrum transmission and reception techniques are described in, e.g.,U.S. Pat. No. 5,022,047 and in U.S. Pat. No. 5,692,007, both of whichare assigned to the assignee of the present invention, and both of whichare hereby incorporated by reference as if set forth fully herein. In apreferred embodiment, the base station 104 and user stations 102 eachtransmit M-ary direct sequence spread spectrum signals using spreadspectrum codes (called “symbol codes”) of 32 chips. Preferably, N databits are transmitted per symbol code, with M different symbol codes areused to represent up to M different data symbols, where M=log₂ N. In apreferred embodiment, thirty-two different symbol codes are used torepresent thirty-two different data symbols, each comprising five bitsof data, and differential phase encoding is used to allow transmissionof a 6th bit of data for each symbol code. Techniques of phase encodingfor transmission of an additional bit of information per symbol code aredescribed in, e.g., U.S. Pat. No. 5,692,007 referred to above.

Because the base header field 553 is positioned first in the basetransmit data frame 551, it “loses” the first bit from the firsttransmitted data symbol (which is transmitted using a differentialencoding technique) because it is used as a phase reference bit. Thusthe base header field 553, which comprises tour data symbols, is 23 bitsin length. The first data symbol comprises five data bits, and thelatter three data symbols each comprises six data bits. Likewise,because the user header field 523 is positioned first in the usertransmit data frame 521, it “loses” the first bit from the firsttransmitted data symbol because it is used as a phase reference bit.Thus the user header field 523, which comprises three symbols, is 17bits in length. The first data symbol comprises five data bits, and thelatter two data symbols each comprises six data bits.

Signaling messages (i.e., messages used for control traffic) may be usedto assist in acquisition and maintenance of a channel from the network.Over-the-air signaling messages may commence with a “message type” dataelement located in a message type field. The message type data elementdefines the format of the rest of the message, and acts as an operationcode to the destination unit (either user station 102 or base station104). Exemplary message types for over-the-air signaling (i.e.; controltraffic) messages appear in Table 9-1 below.

TABLE 9-1 ACK Acknowledge ANS Answer Incoming Call AUT AuthenticationRequest AUR Authentication Response BAI Base Assist Information CIP SetCipher Mode CNC Call Connected CSC Circuit Switch Complete DRGDe-registration Request DRP Drop Incoming Connection HLD Hold ORHOriginating Handover Request ORG Originate Call RCP RegistrationComplete RRQ Registration Request SET Set Services SPR Specific ResponseSYN Synchronize THR Target handover Request TRA Transport MessageThe number of bits of the message type data element used to identify thetype of message depends mainly upon the number of control trafficmessage supported by the system. In a preferred embodiment, the messagetype is 8 bits in length. Additional information needed to process oract upon the message may be contained in other fields in the signalingmessage.

Messages exchanged between the base station 104 and base stationcontroller 105 or other network entities can be mapped to a local orinternal format referred to as “Notes”. Some of these Notes may resemblethe over-the-air signaling messages exchanged between the base station104 and the user station 102, in order to expedite processing of thecontrol traffic messages. The base station controller 105 may act as aprotocol interface whereby signaling messages are translated to a formcompatible with the mobile switching center 112 and/or network.

The general content of certain over-the-air signaling messages that playa role in handover and related functions are set forth in the tablesappearing below. The message content may be viewed as an aspect of“layer three” protocol architecture.

TABLE 10-1 Hold (CT-HLD) Information Element Length in Bits Message Type8 Reserved 152Hold (CT-HLD) control traffic messages can be transmitted either by thebase station 104 or the user station 102. They are generally part of alarger signaling traffic exchange. The user station 102 sends a CT-HLDcontrol traffic message to the base station 104 when the user station102 requires more time to process data and return a result to the basestation 104, or when responding to a CT-HLD control traffic message fromthe base station 104.

TABLE 10-2 Acknowledge (CT-ACK) Information Element Length in BitsMessage Type 8 ACK Response 8 Ack'd Command 8 Ack State 8 Reserved 128

Acknowledge (CT-ACK) control traffic messages can be transmitted byeither the base station 104 or the user station 102. It is not necessarythe every exchange of control traffic messages end with a CT-ACKmessage.

The Ack Response information element of the CT-ACK message contains anacknowledgment response indicator. One of two binary values (i.e., a “0”bit) indicates success, while the other of the two binary values (i.e.,a “1” bit) indicates failure. The Ack'd Command information elementcontains the Message Type of the specific command being acknowledged.The Ack State information element contains the current state of thesystem element (i.e., the base station 104 or user station 102) which istransmitting the acknowledge.

TABLE 10-3 Set Cipher Mode (CT-CIP) Information Element Length in BitsMessage Type 8 Cipher Type 8 Cipher Mode 8 Initialization Vector 64Cause Type 8 Cause 8 Reserved 56

A Set Cipher Mode (CT-CIP) control traffic message is transmitted fromthe base station 104 to the user station 102 to pass pertinent cipheringinformation to the user station 102 and to instruct the user station 102to go into or out of ciphering mode. When the user station 102 receivesthe CT-CIP message, the user station 102 uses the cipher mode parametersto set its ciphering equipment and then switches into or out ofciphering mode. All traffic after the switch to cipher mode will beciphered.

The Cipher Type information element of the CT-CIP message indicates thetype of encryption to be used by the system (e.g., either DCS-1900 orBellcore “C”, for example). The Cipher Mode information elementindicates the encryption mode being requested by the system. TheInitialization Vector information element contains a value to be used inconjunction with other keying information to initialize the encryptionequipment. The Cause information element consists of eight bits of anencoded parameter indicative of what the cause is of an action, and isspecific to a particular control traffic message. For the CT-CIPmessage, the Cause information field can be set to contain a codeindicating such things as set/change cipher or synchronize cipher. TheCause Type information element defines the cause code set to be returnedwhen either the base station 104 or the user station 102 drops aconnection. The Cause Type is stored as an encoded value that identifiesthe code set of the supporting infrastructure. For example, the CauseType information field can be set to contain a value indicating the useof DCS 1900 cause codes or a value indicating the use of BellcoreGeneric “C” cause codes.

TABLE 10-4 Call Origination (CT-ORG) Information Element Length in BitsMessage Type 8 Service Request 32 Key Sequence Number 8 Class 16 CREF 8Reserved 88

The user station 102 sends a Call Originate (CT-ORG) control trafficmessage to the base station 104 to request the placement of an outgoingcall.

The Service Request information element of the CT-ORG message indicatessuch things as data versus voice service, use of CRC and ARQ, symmetryor asymmetry of the channel, whether service resources are beingrequested, and frame rate, for example. The Key Sequence Numberinformation element is used to generate a communication key in both thebase station 104 and the user station 102 without having to explicitlypass the key over the air. The Class information element specifies someof the operational parameters of the particular type of user station102. The Class information element can be broken down into sub-fields ofClass Type and Class Information. The Class Type sub-field may indicatethe general class of the user station 102 (e.g., DCS1900 or IS-41),while the Class Information sub-field may indicate such things asprotocol or revision level, encryption algorithm, RF power rating, powerclass, continuous or discontinuous transmission, and licensed orunlicensed bandwidth. The Call Reference (“CREF”) information elementspecifies the circuit to which data in a transport message belongs. TheCREF field corresponds to the ISDN Call Reference information element.The CREF information element may contain a value indicating whether thecircuit is, for example, ISDN, DCS1900 or DECT.

TABLE 10-5 Call Connect (CT-CNC) Information Element Length in BitsMessage Type 8 Connection Number 40 Map Type 8 Map 32 Cause Type 8 Cause8 CREF 8 Reserved 48

The Call Connect (CT-CNC) control traffic message may be sent from thebase station 104 to the user station 102 when a call, either incoming oroutgoing, is completed or when an outgoing call from the user station102 is rejected.

The Connection Number information element of the Call Connect messagespecifies the specific network connection which was allocated to carrythe bearer channel of the particular user station 102 from the basestation 104 to the network. Unused nibbles and octets of thisinformation element are filled with “F” hex. The Map information elementdescribes the mapping of time slots to a particular user station 102.The format of the Map element is dependent upon the Map Type informationelement in the same frame. The Map Type information element indicates ifthe frame is a “superframe” (aggregated time slots) or “subframe”(single time slot occurring every N time frames). If a superframe maptype, then each bit in the Map information element corresponds to achannel relative to the current channel. If a subframe map type, the Mapinformation element indicates such things as the submultiplex rate(i.e., the number of frames skipped between transmissions), the framephase (i.e., the number of frames skipped before the firsttransmission), and the channel phase (i.e., the number of time slots orchannels skipped before the first transmission). The Cause and CauseType information elements are as described with respect to the CT-CIPmessage. However, for the CT-CNC message, the Cause information elementindicates whether or not the requested connection has been connected.The CREF information element is the same as described with respect tothe CT-ORG message.

TABLE 10-6 Target Handover Request (CT-THR) Information Element Lengthin Bits Message Type 8 Old Connection Number 40 Service Request 32 KeySequence Number 8 Class 16 Old Base Station ID 32 Old Mobility CountryCode (MCC) 16 Old Mobility Network Code (MNC) 8

The Target Handover Request (CT-THR) control traffic message is sentfrom the user station 102 to the target base station 104 to initiate aterminating handover procedure.

The Old Connection Number information element of the CT-THR messagespecifies the specific network connection which was allocated to carrythe bearer channel of the user station 102 from the old base station 104to the network. Unused nibbles and octets of this information elementare filled with “P” hex. The Service Request, Key Sequence Number andClass information elements are as described with respect to the CT-ORGmessage. The Old Base station ID information element identifies theoriginating base station 104 in a handover. The Old MCC informationelement indicates the mobility country code of the originating basestation in a handover, and the Old MNC information element indicates themobility network code of the originating base station in the handover.

TABLE 10-7 Originating Handover Request (CT-OHR) Information ElementLength in Bits Message Type 8 Base ID 32 Mobility Country Code (MCC) 16Mobility Network Code (MNC) 8 Reserved 56

The Originating Handover Request (CT-OHR) control traffic message issent from the user station 102 to the current base station 104 toinitiate an originating handover procedure.

The Base ID information element uniquely identifies the target basestation 104. The MCC and MNC information elements indicate the mobilitycountry code and the mobility network code, respectively, of the targetbase station 104.

TABLE 10-8 Circuit Switch Complete (CT-CSC) Information Element Lengthin Bits Message Type 8 Handover Reference 48 Map Type 8 Map 32 Reserved56

The Circuit Switch Complete (CT-CSC) control traffic message is sentfrom the old base station 104 to the user station 102 to signal that thenetwork connection is available at the target base station 104. Whensent from the old base station 104, the Map information element will beall zeroes to indicate that there are no longer any slots on the oldbase station 104 for the user station 102 to utilize.

The Handover Reference information element is used to identify aspecific handover process that has already been initiated by anoriginating handover request sequence. In a DCS1900 infrastructuresystem, the handover reference number is assigned by the terminated basestation controller 105. The Map Type and Map information elements are asdescribed with respect to the CT-CNC message.

TABLE 10-9 Terminating Handover Complete (CT-THC) Information ElementLength in Bits Message Type 8 Service Request 32 Key Sequence Number 8Class 16 Handover Reference Number 48 Reserved 48

A Terminating Handover Complete (CT-THC) control traffic message is sentby the user station 102 to the target base station 104 to initiate aterminating handover procedure.

The Service Request, Key Sequence Number, and Class information elementsare as described for the CT-ORG message. The Handover Reference Numberinformation element is as described for the CT-CSC message.

TABLE 10-10 Specific Response (CT-SPR) Information Element Length inBits Message Type 8 Cipher Type 8 Cipher Mode 8 Key Info 64 Class 16Reserved 56

The Specific Response (CT-SPR) control traffic message is sent from theuser station 102 to the base station 104 when the user station 102 islistening for a page and receives a Specific Poll control trafficmessage which contains the user station's PID and which is marked as a“paging” Specific Poll message.

The Cipher Type and Cipher Mode information elements are as describedfor the CT-CIP message. The Key Info information element contains avalue to be used in conjunction with other keying information toinitialize the encryption equipment, and the contents of this fielddepend upon the specific type of supporting infrastructure (e.g.,DCS1900). The Class information element is as described for the CT-ORGmessage.

TABLE 10-11 Set Service (CT-SET) (user to base) Information ElementLength in Bits Message Type 8 Reserved 80 Map Type 8 Map 32 ServiceRequest 32

The user station 102 sends a Set Service (CT-SET) control trafficmessage to the base station 104 when the user station 102 desires tochange the characteristics of the over-the-air service.

The Map Type and Map information elements are as described for theCT-CNC message. The Service Request information element is as describedwith respect to the CT-ORG message.

TABLE 10-12 Set Service (CT-SET) (base to user) Information ElementLength in Bits Message Type 8 Cause Type 8 Cause 8 Connect Number 40Reserved 24 Map Type 8 Map 32 Service Request 32

The base station 104 sends a Set Service (CT-SET) control trafficmessage to the user station 102 when the base station 104 wishes tochanges the characteristics of over-the-air service.

The Connection Number, Map Type and Map information elements of theCT-SET message are as described for the CT-CNC message. The Cause Typeand Cause information elements are as described for the CT-CIP message.However, the Cause information element for the CT-SET message indicateswhether the link was successfully established or else failed.

TABLE 10-13 Release (CT-REL) Information Element Length in Bits MessageType 8 Cause Type 8 Cause 8 Reserved 136

The Release (CT-REL) control traffic message is sent by the base station104 to the user station 102 when the network releases the connection inprogress or during link setup. The Cause Type and Cause informationelements are as described for the CT-CIP message. However, the Causeinformation element for the CT-REL message indicates whether the releasewas initiated by the network, or whether an authentication rejectionoccurred.

TABLE 10-14 Base Assist (CT-BAM) Information Element Length in BitsMessage Type 8 Base Assist Information 152

The Base Assist (CT-BAM) control traffic message is sent by the basestation 104 to the user station 102 whenever the base station 104desires to pass information to the user station 102 which will help theuser station 102 in making well informed decisions. The contents of theBase Assist information element vary depending upon the circumstances.

TABLE 10-15 Transport (CT-TRA) Information Element Length in BitsMessage Type 8 Transport Data 56

The Transport (CT-TRA) control traffic message is used for transportingdata between the base station 104 and the user station 102 on thecircuit specified by the Call Reference Number (CREF). The contents ofthe Transport Data information element varies depending upon theapplication, and generally constitutes application level data.

Transport control traffic messages differ from other control trafficmessages in that the Message Type information element containsadditional information. The format of the Message Type field forTransport messages is as follows:

TABLE 10-15A Message Type Header for Transport Header Element BitPosition(s) Transport Bit 8 ACK/NAK 7 Message Number 6 CREF 1-5

The Transport Bit indicates whether or not the message is a Transportmessage. The ACK/NAK bit indicates whether or not the sender receivedthe last message without error. The Message Number bit indicates themessage number (0 or 1) of the current message, and should alternate foreach message sent by the same entity. The Call Reference identifies thecall.

The values passed as part of Message Type information element allow thereceiving entity (base station 104 or user station 102) to correct amessage error. In one embodiment, the following steps are undertaken toattempt to correct a message:

-   -   1) The receiving entity compares the Message Number of the        received message with the Message Number of the previously        received message. If they are the same, the receiving entity        ignores the new message.    -   2) The receiving entity checks the ACK/NAK field of the received        message. If the value is NAK, it resends the old packet, and if        the value is ACK, it sends the new packet.    -   3) Each sender complements the message number each time a new        packet is sent.    -   4) If the receiving entity receives a message with a FCW error,        or if it does not receive a message at all, it resends the old        packet with the NAK bit set.

In addition to the above messages, various signaling messages may beused between the base station and the network to convey information atthe call control entity level. Exemplary call control messages includethose appearing in Table 9-2 below.

TABLE 9-2 Call Establishment Messages Direction CC-SETUP BothCC-INFOrmation Both CC-CALL-PROCeeding Network -> User CC-ALERTING BothCC-PROGress Network -> User CC-CONNECT Both CC-CONNECT-ACKnowledge BothCC-EMERGENCY-SETUP User -> Network CC-CALL-CONFIRMED User -> NetworkCall Release Messages Direction CC-DISConnect Both CC-RELEASE BothCC-RELEASE-COMplete Both Call Related Supplementary Services DirectionHOLD User -> Network HOLD-ACKnowledge Network -> User HOLD-REJECTNetwork -> User RETRIEVE User -> Network RETRIEVE-ACKnowledge Network ->User RETRIEVE-REJECT Network -> User DTMF Interaction DirectionStart-DTMF User -> Network Stop-DTMF User -> Network Start-DTMF-ACKNetwork -> User Stop-DTMF-Ack Network -> User Start-DTMF-Reject Network-> User

The interplay among the various entities involved in the transfer ofsignaling messages and other information may be better understood byreference to FIG. 21, which depicts a preferred system protocolarchitecture. As illustrated in FIG. 21, a preferred user station 102(designated “MS” in FIG. 21) includes a Communication Management (“CM”)entity, a Mobility Management (“MM”) entity, and a Radio Resources(“RR”) entity, among others. The CM and MM entities of the user station102 communicate with their counterparts at a mobile switching center 112(designated “MSC” in FIG. 21), via links connected across a base station104 (designated “BS” in FIG. 21) and base station controller 105(designated “BSC” in FIG. 21). The various types of signaling interfacesof a preferred embodiment are shown in FIG. 21 by the arrows connectinglike entities.

The “Layer 3” protocol exchange between the mobile switching center 112and the base station controller 105 is characterized by the BSSMAPprotocol. The “Layer 3” protocol exchange between the mobile switchingcenter 112 and the user station 102 is characterized by the DirectTransfer Application Part (DTAP). DTAP is further divided into twological sublayers, defined by the CM and MM entities described above.The CM includes call control and supplementary services management,including short message service.

Most DTAP messages are not interpreted by the base station controller105 or the base station 104. Rather, they are transferred to the networkby the mobile switching center 112 over a network interface (such as theGSM A-interface). Most radio resource (RR) messages are mapped to BSSMAPmessages at the base station controller 112. However, some of thesemessages are interpreted by the base station 104 (e.g., pagingmessages). The control management (CM) part of the protocol is addressedby an ISDN based CM message set, referred to as IGCC (ISDN Generic CallControl). Control management messages from the user station 102 aredirectly transferred to the network over the interface at the mobileswitching center 112. Interface adapters at the user station 102 and thebase station controller 105 segment control management (i.e., IGCC)messages into packets, which are individually transported between theuser station 102 and the base station 104 via CT-TRA Control Trafficmessages and between the base station 104 and base station controller105 via Transport Notes. Notes are transmitted over a CCITT ISDN datalink (Q.920/Q.921) The interface adapters at the user station 102 andbase station controller 105 are responsible for ensuring that thepackets are sequenced properly and the entire IGCC message is errorfree.

Radio resource (RR) messages and mobility management (MM) messages takethe form of internal Notes between the base station controller 105 andbase station 104, and are mapped at the base station to over-the-airmessages when sent to the user station 102.

Exemplary message flow diagrams for various calling functions are shownin FIGS. 9, 10, 11A-11C and 12A-12B. While generally described withrespect to features referenced in the FIG. 3 embodiment, they have equalapplicability to the FIG. 6 embodiment.

An exemplary message flow diagram for call origination from a userstation 102 is shown in FIG. 9. In FIG. 9 messages are designated byarrows (1) between a user station 102 (abbreviated “MS”) and a basestation 104 (abbreviated “BS”), (2) between the base station 104 and abase station controller 105 (abbreviated “BSC”), and (3) between thebase station controller 105 and a mobile switching center 112(abbreviated “MSC”). The MSC 112 generally acts a switch controllingaccess to the network 106 (as shown, e.g., in FIG. 2). Control trafficmessages between the user station 102 and the base station 104 aretypically preceded by the initials “CT” in FIG. 9. The steps numbered 1through 17 associated with the arrows appearing in FIG. 9 are explainedbelow:

-   -   1. A user station application sends a call originate request to        the user, station 102 over-the-air controller.    -   2. The user station 102 seizes an available time slot (such as,        for example, time slot 302 in FIG. 3 or virtual time slot 618 in        FIG. 6) in accordance with the protocol shown in FIG. 4 and/or        4A. If no time slot is acquired, the user station 102 times out        and attempts to register and then acquire a time slot on another        base station 104.    -   3. Upon successful time slot acquisition, the user station 102        responds to the specific poll message 402 with an ORIGINATE        control traffic (CT-ORG) message. The CT-ORG message includes        circuit reference (CREF) information.    -   4. The base station 104 responds to the CT-ORG message by        sending a control traffic acknowledgment (CT-ACK) message back        to the user station 102. (If no CT-ACK message is received from        the base.station 104, the link is dropped and the user station        102 attempts to originate a call on another base station 104.)        The base station 104 assigns time slots and terrestrial bearer        channels to support the service request (if possible) and then        sends a Setup Link NOTE containing the terrestrial bearer        information to the base station controller 112. If, however, the        base station 104 is unable to assign time slots or bearer        channels, it returns a control traffic setup (CT-SET) message        indicating is a failure status to the user station 102, and no        Setup Link NOTE is sent to the base station controller 105. The        user station 102 then attempts to originate a call using another        base station 104.    -   5. When the base station controller 105 receives the Setup Link        NOTE, it builds a signaling connection control part (SCCP)        channel for the user station 102 based on the PID of the user        station 102. The base station controller 105 also retains the        Setup Link NOTE parameters for use in a later step. If        construction of the SCCP channel fails, a Connect Link NOTE is        sent to the base station 104 indicating a failure status. The        base station 104 then responds by sending a control traffic Set        Service (CT-SET) message to the user station 102 indicating the        link failure (see Step 7 below). The link failure is        communicated to the user station application via a Connect        message (see Step 10 below).    -   6. After the CT-ACK message is received at the user station 102,        the user station 102 and the base station 104 enter a HOLD        sequence while waiting for a link to be established between the        base station 104 and the base station controller 105. During        this sequence, the user station 102 and base station 104        periodically exchange control traffic HOLD (CT-HLD) messages. If        the base station 104 and/or user station 102 unexpectedly stops        receiving CT-HLD messages or the user station 102 does not        subsequently receive a CT-SET message from the base station 104        after the CT-ORG message has been sent, then the base station        104 disconnects the link from the base station controller 105        using call clearing procedures, and the user station 102 and        base station 104 attempt lost link recovery. If the lost link        recovery procedure is successful, then call origination from the        user station 102 is re-initiated.    -   7, When the SCCP channel is constructed, the base station        controller 105 sends a Connect Link NOTE to the base station        104. The Connect Link NOTE includes status information from the        base station controller 105.    -   8. The base station 104 then sends a control traffic SET SERVICE        (CT-SET) message to the user station 102. This message defines        the slot structure (i.e., mapping) to be used by the user        station 102. The CT-SET message includes the status information        contained in the Connect Link message received from the base        station controller 105.    -   9. The user station 102 acknowledges receipt of the CT-SET        message by responding with a control traffic acknowledgment        (CT-ACK) message. If the base station 104 does not receive the        CT-ACK message, then the base station 104 disconnects the link        from the base station controller 105 and attempts lost link        recovery in a manner similar to that described with respect to        Step 6 above.    -   10. The user station 102 responds to the CT-SET message by        sending a Connect message to the user station application. The        Connect message indicates to the user station application        whether or not the control link has been established.    -   11. The user station 102 and base station 104 then enter a HOLD        sequence by exchanging control traffic hold (CT-HLD) messages, a        condition which is sustained as long as no IGCC Setup message        traffic is being transported from or to the user station        application (see Step 12). If the communication link is lost        between the base station 104 and the user station 102 after the        Connect message has been sent to the user station application,        then the base station disconnects the link from the base station        controller 105 using call clearing procedures. The user station        102 sends a Link Lost message to the user station application.        Any message from the user station application that does not        initiate a new operation will cause the user station 102 to        respond with another Link Lost message.    -   12. The user station application sends an ISDN generic call        control (“IGCC”) Setup message through the user station 102 and        base station 104 to the base station controller 105 via control        traffic Transport (CT-TRA) messages and Transport NOTES. The        user station 102 and base station 104 return to the hold        sequence whenever no IGCC messages are available for transport.    -   13. The base station controller 105 sends a Service Request        message to the mobile switching center 112 via a Complete L3        Info DTAP message.    -   14. The mobile switching center 112 responds to the base station        controller 105 with a call management (CM) Service Accept DTAP        message.    -   15. The user station application completes the call setup via        end-to-end IGCC based call procedures.    -   16. Once the IGCC Call Control has the call established, the        user station application sends a Begin Traffic request to the        user station 102,    -   17. The system enters normal traffic mode, and the conversation        (if voice) or other data path is stable.

An exemplary message flow diagram for processing a call originating fromthe network and terminating at a user station 102 is shown in FIG. 10.In FIG. 10 are shown abstractly by arrows, similar to FIG. 9, messagesbetween a user station 102 (abbreviated “MS”) and a base station 104(abbreviated “BS”), between the base station 104 and a base stationcontroller 105 (abbreviated “BSC”), and between the base stationcontroller 105 and a mobile switching center 112 (abbreviated “MSC”).Control traffic messages between the user station 102 and the basestation 104 are typically preceded by the initials “CT” in FIG. 10. Thesteps numbered 1 through 33 associated with the arrows appearing in FIG.10 are explained below:

-   -   1. The mobile switching center 112 originates a call by sending        a BSSMAP PAGING message to the base station controller 105. The        BSSMAP PAGING message is sent as a “connectionless” message to        the base station controller 105, and includes the personal        identifier (PID) of the user station 102 being paged.    -   2. The base station controller 105 searches its Location        Register (LR) for the entry of an international mobile station        identifier (IMSI) matching the PID sent in the BSSMAP PAGING        message. If the matching user station PID is not found, then the        base station controller 105 does not respond to the mobile        switching center 112 and the call is dropped.    -   3. If the base station controller 105 identifies the appropriate        entry, the base station controller 105 sends a Page NOTE to the        base station 104 associated with the entry. The service request        type in the Page NOTE is set to zero (indicating that a NULL        service is being requested). The Page Note allows the base        station 104 to page the user station 102 without having to set        up a specific call.    -   4. When the base station 104 receives a Page NOTE with a NULL        service request, the base station 104 sends a SPECIFIC POLL        message (e.g., specific poll message 402 in FIG. 4) with service        type set to zero (indicating a NULL request). The base station        104 queues the Page NOTES and sends SPECIFIC POLL messages        corresponding to the Page NOTES on a cyclic basis. If there are        more Page NOTES than there are available unused time slots (time        slots), the base station 104 sends the SPECIFIC POLL messages        sequentially on the available time slots. Consequently, the        SPECIFIC POLL messages may be spread out over many time frames        (polling loops). The base station 104 continues to issue the        SPECIFIC POLL message until either the user station 102        responds, or until predetermined time period T_(page) associated        with the Page NOTE expires (as measured by an internal timer).    -   5. The user station 102 alternates between an inactive or sleep        state and an active state with a predetermined duty cycle. When        the user station 102 wakes up, it scans all SPECIFIC POLL        messages from the base station 104 upon which it is registered.        In one embodiment, the user station 102 scans until the same        user station PID is seen twice. If an user station 102 does not        see a SPECIFIC POLL containing its user station PID (or does not        see the user station PID twice, if applicable), then after a        predetermined monitoring time the user station 102 returns to        sleep for a time period dictated by its duty cycle.    -   6. When user station 102 receives a SPECIFIC POLL control        traffic message containing its user station PID, the user        station 102 responds with a SPECIFIC POLL RESPONSE control        traffic (CT-SPR) message (e.g., specific poll response 405 in        FIG. 4). If no SPECIFIC POLL is seen by the user station 102        with its user station PID, then it goes back to sleep for a        predetermined time period.    -   7. When the base station 104 receives the SPECIFIC POLL RESPONSE        control traffic message from user station 102 having the        matching user station PID, the base station 104 returns an        acknowledgment control traffic (CT-ACK) message to the user        station 102, and sends a Page Response NOTE to the base station        controller 105. If the base station 104 does not receive a        SPECIFIC POLL RESPONSE control traffic message from the user        station 102, it does not send a Page Response NOTE, and the call        is dropped.    -   8. The base station 104 and user station 102 then eater a slot        maintenance mode in which they pass HOLD control traffic        (CT-HLD) messages back and forth. If the base station 104 or        user station 102 unexpectedly stops receiving CT-HLD control        traffic messages, then the base station 104 and user station 102        attempt lost link recovery. If lost link recovery fails, then        the call is dropped.    -   9. Upon receipt of the Page Response NOTE from the base station        104, the base station controller 105 builds an SCCP circuit to        the mobile switching center 112 and associates the SCCP circuit        with the user station PID. If the base station 104 does not        receive a Page Response NOTE, the call is dropped. Further, if        construction of the SCCP circuit fails, the Setup Link NOTE so        indicates. The base station 104 responds by sending a control        traffic Set Service (CT-SET) message to the user station 102        indicating the failure.    -   10. Once the base station controller 105 has built an SCCP        circuit to the mobile switching center 112, the base station        controller 105 sends a BSSMAP Paging Response message to the        mobile switching center 112 over the SCCP circuit.    -   11. The mobile switching center 112 then sends a DTAP Setup        message to the base station controller 105, using the SCCP        circuit associated with the user station's PID.    -   12. When the base station controller 105 receives the DTAP Setup        Message from the mobile switching center 112, the base station        controller 105 sends a Setup Link NOTE to the base station 104        communicating with the particular user station 102.    -   13. When the base station 104 receives the Setup Link NOTE from        the base station controller 105, the base station 104 assigns        radio resources (e.g., time slots) to satisfy the service        request data element. The base station 104 then sends the base        station controller 105 a Service information NOTE detailing the        bearer channels assigned to this call. If the base station 104        does not receive a Setup Link NOTE, then call clearing        procedures are initiated. If the base station 104 cannot supply        the resources requested by the base station controller 105, this        fact is indicated in a “result” field of the Service Information        NOTE.    -   14. The base station 104 communicates the service desired and        the air resources necessary to support the service to the user        station 102 using a Set Service (CT-SET) control traffic        message.    -   15. The user station 102 responds to the CT-SET message by        sending an acknowledge control traffic (CT-ACK) message back to        the base station 104 and sending a Setup Link message to the        user station application. If no CT-ACK message is received by        the base station 104, call clearing procedures are initiated.    -   16. The user station application responds to the Setup Link        message with a Connect Link message.    -   17. The base station 104 and user station 102 then enter a slot        maintenance mode in which they pass HOLD control traffic        (CT-HLD) messages back and forth. If the base station 104 or        user station 102 unexpectedly stops receiving CT-HLD control        traffic messages, the base station 104 and user station 102        attempt lost link recovery. If lost link recovery fails, call        clearing procedures are initiated.    -   18. After the user station 102 configures itself to provide the        requested service and receives the Connect Link message from the        user station application, the user station 102 responds with a        CONNECT LINK (CT-CNL) control traffic message with the        “response” field set to indicate a successful connection. If the        user station 102 cannot satisfy the service request, the user        station 102 replies with the “response” field set to indicate        failure. If the user station 102 does not receive a CT-ACK        message, the user station 102 disconnects the link according to        call clearing procedures, and the call is dropped.    -   19. When the base station 104 receives the CT-CNL control        traffic message, it returns a control traffic acknowledgment        (CT-ACK) message to the user station 102. Once the base station        104 has allocated all necessary channel resources, it sends a        Connect Link NOTE to the base station controller 105.    -   20. The user station 102 and base station 104 enter a hold        sequence in which they exchange CT-HLD messages to maintain the        over-the-air channel, If the base station 104 or user station        102 unexpectedly stops receiving CT-HLD control traffic        messages, the base station 104 and user station 102 attempt lost        link recovery. If lost link recovery fails, call clearing        procedures are initiated,    -   21. The base station controller 105 responds to the Connect Link        NOTE by returning a Connect Link NOTE back to the base station        104. The Connect Link NOTE from the base station controller 105        contains a connection number for the call.    -   22. Upon receiving the Connect Link NOTE from the base station        controller 105, the base station 104 sends a CONNECT COMPLETE        (CT-CNC) control traffic message to the user station 102. The        CT-CNC message communicates the connection number for the call        to the user station 102. If the user station 102 does not        receive the CT-CNC message, or the base station 104 does not        receive a CT-ACK message in response, lost link recovery is        attempted. If lost link recovery fails, call clearing procedures        are initiated.    -   23. The user station 102, as suggested in step 22, acknowledges        the CT-CNC control traffic message with a control traffic        acknowledgment (CT-ACK) message.    -   24. Upon receiving the CT-ACK control traffic message, the base        station 104 sends an Acknowledge NOTE to the base station        controller 105, with the command argument set to “Connect Link,”        to indicate completion of the link.    -   25. The base station 104 and user station 102 then enter a slot        maintenance mode in which the pass HOLD (CT-HLD) control traffic        messages back and forth. This sequence is sustained as long as        no other message traffic is being transported to or from the        user station application. If the base station 104 or user        station 102 unexpectedly stops receiving CT-HLD control traffic        messages, the base station 104 and user station 102 attempt lost        link recovery. If lost link recovery fails, call clearing        procedures are initiated.    -   26. When the base station controller 105 receives the        Acknowledge NOTE from the base station 104, the base station        controller 105 initiates ISDN generic call control (IGCC)        message traffic that sets up the link with the user station 102.        The base station controller 105 uses the information from the        DTAP Setup message (see Step 11) during the IGCC setup process.    -   27. Upon completion of the IGCC setup, an IGCC Call Confirmed        message is sent from the user station application to the base        station controller 105.    -   28. Once the call is confirmed between the user station        application and the base station controller 105, the base        station controller 105 sends a DTAP Call Confirmed message to        the mobile switching center 112 on the SCCP circuit associated        with the user station's PID.    -   29. In response to the DTAP Call Confirmed message, the mobile        switching center 112 sends the base station controller 105 a        BSSMAP Assignment Command message.    -   30. When the base station controller 105 receives the BSSMAP        Assignment Command message from the mobile switching center 112,        the base station controller 105 connects the circuit described        by the Circuit ID code to the        base-station-to-base-station-controller circuit described by the        map in the Service Information NOTE. Once the Assignment Command        message has been received from the mobile switching center 112        and the Connect Link NOTE has been received from the base        station 104, the base station controller 105 sends the mobile        switching center 112 a BSSMAP Assignment Complete message on the        SCCP circuit associated with the user station's PID.    -   31. When the mobile switching center 112 receives the BSSMAP        Assignment Complete message from the base station controller        105, the mobile switching center 112 initiates IGCC end-to-end        call control traffic.    -   32. When the connection is complete and the user station        application is ready to accept/Bend data, the user station        application sends a Begin Traffic message to the user station        102.    -   33. The system then enters normal traffic mode, and the        conversation is stable.

FIGS. 11A-11C and 12A-12B are message flow diagrams for an intra-clusterhandover and an inter-cluster handover, respectively. These message flowdiagrams may be explained with reference to FIG. 19, which illustrates aparticular deployment of base stations in clusters. In FIG. 19, a mobileswitching center 112 120 is connected to a plurality of base stationcontrollers 105 (also referred to as cluster controllers). Each basestation controller 105 is in turn connected to a plurality of basestations 104. The base stations 104 are organized into logical groups ofclusters 121, such that each cluster 121 of base stations 104 isconnected to a single base station controller 105. A cluster 121 of basestations 104 need not be geographically adjacent; rather, the cluster121 comprises a logical group of base stations 104 regardless of theirgeographical proximity.

As used herein, an intra-cluster handover is one in which a user station102 transfers communication from the current base station 104 to a newbase station 104 in the same cluster 121 (i.e., in a cluster 121 that isserviced by the same base station controller 105), and an inter-clusterhandover is one in which the user station 102 transfers communicationfrom the current base station 104 to a new base station 104 in adifferent cluster 121 (i.e., in a cluster 121 that is serviced by adifferent base station controller 105).

An exemplary message flow diagram for an intra-cluster handover is shownin FIGS. 11A-11C. As described in more detail below, FIG. 11B relates tothe case in which the link with the old base station is maintained,while FIG. 11C relates to the case in which the link with the old basestation is lost. In FIGS. 11A-11C, similar to FIGS. 9 and 10,transmitted messages are designated by arrows between a user station102, a current base station 104 (denoted “old BS” or “BS1”), a targetbase station 104 (denoted “New BS” or “BS2”), a base station controller105, and a mobile switching center 112. Control traffic messages betweenthe user station 102 and either base station 104 are typically precededby the initials “CT”. The steps numbered 1 through 22 associated withthe arrows appearing in FIGS. 11A-11C are explained below:

-   -   1. The user station 102 starts in normal stable traffic with the        current base station 104 (BS1).    -   2. The user station 102 monitors a received signal strength        indication (RSSI). Eventually, the RSSI for the current link        drops below a first threshold value L_(look) (i.e., the        threshold value below which the user station 102 begins to        search for a new base station 104).    -   3. During a portion of the time frame 301 that the user station        102 does not need to maintain communication in its assigned time        slots with the current base station BS1, the user station 102        switches to the frequency (e.g. F1, F2 or F3) and/or code (e.g.,        C1, C2, C3, C4, C5 or C6) of one of the surrounding base        stations 104, as specified by a surrounding base station table,        and measures the RSSI of that base station 104 by observing any        traffic from the base station 104. The user station 102 also        records the current utilization field from the header of the        base station 104 traffic messages (e.g., CU field 809 of FIG.        8A). If the message observed is a GENERAL POLL message, then the        user station 102 also records the slot quality, base ID, base        station controller ID (BSC ID), service provider, zone and        facility of the candidate base station 104. The user station 102        uses this information to calculate a preference value for the        candidate base station 104 and sorts the entry into a table of        preferred base stations.    -   4. When the RSSI of the link to the current base station BS1        drops below a second threshold level L_(ho) (i.e., the threshold        below which handover is appropriate), the user station 102        selects the highest preference base station 104 as the target        base station 104 (BS2). If the observed time slot 302 at the        target base station BS2 had contained a GENERAL POLL message,        then the user station 102 examines the BSC ID of the target base        station BS2. If the BSC ID is not the same as that of the        current base station BS1 (i.e., the current and target base        stations are connected to different base station controllers        105), then the user station 102 executes an inter-cluster        handover (see FIGS. 12A-12B). Similarly, the user station 102        examines the zone of the target base station BS2, and if the        zone is not the same as the zone of the old base station BS1,        then the user station 102 will commence execution of an        inter-cluster handover (see FIGS. 12A-12B). Otherwise, if the        BSC ID is the same for the current and target base stations and        the zone for both is also the same, then the user station 102        continues with an intra-cluster handover. If the observed time        slot did not contain a GENERAL POLL message, then the user        station 102 attempts to locate a time slot that has a GENERAL        POLL message. The user station 102 can potentially look at all        of the time slots in which it is not presently communicating        and, if desired, can even skip a transmission on its current        time slot to check the same location time slot on the target        base station BS2 for a GENERAL POLL message.    -   5. The user station 102 acquires the observed time slot of the        target base station BS2. The user station 102 does this by        searching for a GENERAL POLL message from the target base        station BS2, and responding with a GENERAL RESPONSE message to        the GENERAL POLL message. If the user station 102 has not        already examined the BSC ID of the target base station BS2, it        does so at this point. If the BSC ID and zone match those of the        old base station BS1, then the user station can perform an        intra-cluster handover utilizing the target base station BS2.        Otherwise, if either the zone of the BSC ID of the target base        station BS2 does not match that of the old base station BS1,        then the user station 102 does not respond to the SPECIFIC POLL        message, but instead executes an inter-cluster handover (see        FIGS. 12A-12B).    -   6. Assuming an intra-cluster handover is to be performed, the        user station 102 and old base station BS1 maintain traffic        communication over the old link if possible. If not possible,        the old link is dropped.    -   7. In response to the SPECIFIC POLL control traffic message from        the target base station BS2, the user station 102 returns a        TERMINATING HANDOFF REQUEST (CT-THE) control traffic message.    -   8. If the target base station BS2 will accept the handover, the        target base station BS2 responds with a BASE ASSIST (CT-BAM)        control traffic message. The CT-BAM message contains a list of        surrounding base stations 104 which the user station 102 can        monitor for future handovers. The user station 102 responds with        a HOLD (CT-HLD) control traffic message and sets an internal        user station handover timer. The handover is at this stage        considered to be committed in the sense that the user station        102 cannot attempt a new handover until this attempt is        completed. If the user station 102 does not receive a CT-BAM        message from the target base station BS2, the user station 102        will attempt to hand off to the next most preferable base        station 104 it found in Step 3 above. If there are no other        suitable base stations 104, the user station 102 will proceed        with call clearing.    -   9. If the target base station BS2 has accepted the handover, it        sets a base station handover timer and sends a Terminating        Handoff Note to the base station controller 105.    -   10. The base station controller 105 switches the user station        102 from the old base station BS1 to the new base station BS2.        Specifically, the base station controller 105 switches the        circuit represented by the Circuit ID code associated with the        user station 102 as identified in the local registration (LR) of        the base station controller 105 from the old circuit (described        by the connection number) to a new circuit at the target base        station BS2 as described by a Bearer Map in the Terminating        Handoff Request NOTE. The base station controller 105 thereafter        associates the user station 102 with its new location. The base        station controller 105 updates the contents of the location        register (LR) to reflect the new location of the user station        102.    -   11. In response to the CT-BAM message from the base station 104,        the user station sends a control traffic acknowledge (CT-ACK)        message to the base station 104 to acknowledge receipt of the        surrounding base station list.

If the link between the user station 102 and the old base station BS1can be maintained, the following steps are then carried out, inaccordance with the call flow diagram of FIG. 11B;

-   -   12. After receiving a CT-ACK message from the user station 102,        the target base station BS2 starts issuing SPECIFIC POLL        messages targeted for the user station 102 (using the user        station PID), go that the user station 102 can re-acquire the        link on the target base station BS2.    -   13. When the base station controller 105 completes its circuit        switch, the base station controller 105 sends the target base        station BS2 a Circuit Switch Complete NOTE. In one embodiment,        the Circuit Switch Complete NOTE contains no ciphering        information.    -   14. The base station controller 105 also sends the old base        station BS1 a Circuit switch Complete NOTE. When the old base        station BS1 receives the Circuit Switch Complete NOTE, the old        base station BS1 sends a CIRCUIT SWITCH COMPLETE (CT-CSC)        control traffic message to the user station 102. The old base        station BS1 then clears all tables and circuits related to the        call.    -   15. The base station controller 105 then sends a BSSMAP HANDOVER        PERFORMED message to the mobile switching center 112.    -   16. When the user station 102 receives the CT-CSC control        traffic message, the user station 102 responds by switching to        the frequency and code of the new base station BS2. The user        station 102 then searches for a SPECIFIC POLL message with the        PID field matching the PID of the user station 102. When the        user station 102 finds the appropriate SPECIFIC POLL message, it        responds with a HOLD (CT-HLD) control traffic message. It the        user station 102 loses the link to the old base station BS1        before receiving the CT-CSC message, the user station 102 will        switch to the target base station BS2 and respond to the        SPECIFIC POLL message. If the user station 102 is unable to find        a SPECIFIC POLL message with the proper PID on the target base        station BS2, then the call is lost, and the user station        proceeds with call clearing    -   17. When the target base station BS2 sees a CT-HLD message from        the user station 102 and has received the Circuit Switch        Complete NOTE from the base station controller 105, the target        base station BS2 sends a CIRCUIT SWITCH COMPLETE (CT-CSC)        control traffic message to the user station 102.    -   18. When the user station 102 receives the CT-CSC message from        the target base station BS2, the user station 102 cancels its        internal user station handover timer, and responds with bearer        traffic messages. If the user station's handover timer expires        before bearer traffic is received, the connection is lost, and        the user station 102 will proceed with call clearing.    -   19. When the target base station BS2 receives the bearer traffic        messages from the user station 102, the target base station BS2        cancels its base station handover timer and switches into        traffic mode. If the base station handover timer expires before        bearer traffic is receives, then the connection is assumed lost,        and the base station BS2 will proceed with call clearing.    -   20. A stable bearer channel has been established with the new        base station BS2. Handover is complete.

Steps 12-19 above assume that the link between the user station 102 andthe old base station BS1 is maintained during handover. If, however, thelink between the user station 102 and the old base station BS1 is lost,then the following steps are carried out to complete the intra-clusterhandover, in accordance with the call flow diagram of FIG. 11C:

-   -   21. If the user station 102 loses the link with the old base        station BS1 before it receives the CT-CSC control traffic        message, then the user station 102 switches to the frequency of        the target base station BS2 and searches for a SPECIFIC POLL        message having a PID field matching the PID of the user station        102. When the user station 102 finds the appropriate SPECIFIC        POLL message, the user station 102 responds with a HOLD (CT-HLD)        control traffic message. If the user station 102 is unable to        find a SPECIFIC POLL on the target base station BS2, then the        call is assumed lost, and the user station 102 proceeds with        call clearing.    -   22. If the target base station BS2 receives a response to its        SPECIFIC POLL message from the user station 102 before the        target base station BS2 has received the Circuit Switch Complete        NOTE from the base station controller 105, the target base        station BS2 responds to the CT-HLD messages from the user        station 102 with CT-HLD messages, in an alternating fashion.    -   23. When the base station controller 105 completes its switch,        it sends the target base station BS2 a Circuit Switch Complete        NOTE.    -   24. When the target base station BS2 receives the Circuit Switch        Complete NOTE from the base station controller 105, the target        base station BS2 sends a CIRCUIT SWITCH COMPLETE (CT-CSC)        control traffic message to the user station 102.    -   25. The base station controller then sends the old base station        BS1 a Circuit Switch Complete NOTE. In one embodiment, the        Circuit Switch Complete NOTE contains no ciphering information.    -   26. The base station controller 105 then sends a BSSMAP ANDOVER        PERFORMED message to the mobile switching center 112.    -   27. When the old base station BS1 receives the Circuit Switch        Complete NOTE, the old base station BS1 sends a CT-CSC control        traffic message to the user station 102. (The user station 102        will not see this message because it has lost the link to the        old base station BS1.) The old base station BS1 then clears all        tables and circuits related to the call.    -   28. When the user station 102 receives the CT-CSC message from        the target base station BS2, the user station 102 responds with        bearer traffic, and cancels its internal user station handover        timer.    -   29. When the target base station BS2 receives a bearer traffic        response from the user station 102, the target base station BS2        cancels its base station handover timer, and switches into a        traffic mode. A stable bearer channel has been established at        this point.

The foregoing description pertains to intra-cluster handovers. A systemin accordance with a preferred embodiment is also capable of performinginter-cluster handovers. Exemplary message flow diagrams for aninter-cluster handover is shown in FIGS. 12A-12B. In FIGS. 12A-12B,similar to FIGS. 9-11, messages are designated by arrows between a userstation 102, a current base station 104 (denoted “Old BS” or “BS1”), atarget base station 104 (denoted “New BS” or “BS2”), a current basestation controller 105 (denoted “Old BSC” or “BSC1”), a target basestation controller 105 (denoted “New BSC” or “BSC2”), and a mobileswitching center 112. Control traffic messages between the user station102 and either base station 104 are typically preceded by the initials“CT”. The steps numbered 1 through 33 associated with the arrowsappearing in FIGS. 12A-12B are explained below (with steps 1 through 4being identical to those for an intra-cluster handover):

-   -   1. The user station 102 starts in normal stable traffic with the        current base station 104 (BS1).    -   2. The user station 102 monitors a received signal strength        indication (RSSI). Eventually, the RSSI for the current link        drops below a first threshold value L_(look) (i.e., the        threshold value below which the user station 102 begins to        search for a new base station 104).    -   3. During a portion of the time frame 301 that the user station        102 does not need to maintain communication in its assigned time        slots with the current base station BS1, the user station 102        switches to the frequency (e.g., F1, F2 or F3, as shown in the        example of FIG. 1A) and/or code (e.g., C1, C2, C3, C4, C5, C6 or        C7, as shown in the example of FIG. 1A) of one of the        surrounding base stations 104, as specified by a surrounding        base station table, and measures the RSSI of that base station        104 by observing any traffic from the base station 104. The user        station 102 also records the current utilization field from the        header of the base station 104 traffic messages (e.g., CU field        809 of FIG. 8A). If the message observed is a GENERAL POLL        message, then the user station 102 also records the slot        quality, base ID, base station controller ID (BSC ID), service        provider, zone and facility of the candidate base station 104.        The user station 102 uses this information to calculate a        preference value for the candidate base station 104 and sorts        the entry into a preferred base station table.    -   4. When the RSSI of the link to the current base station BS1        drops below a second threshold level L_(ho) (i.e., the threshold        below which handover is appropriate), the user station 102        selects the highest preference base station 104 as the target        base station 104 (BS2). If the observed time slot 302 at the        target base station BS2 had contained a GENERAL POLL message,        then the user station 102 examines the BSC ID of the target base        station BS2. If the BSC ID is not the same as that of the        current base station BS1 (i.e., the current and target base        stations are connected to different base station controllers        105), then the user station 102 executes an inter-cluster        handover, as described in further detail in the steps below.        Similarly, the user station 102 examines the zone of the target        base station BS2, and if the zone is not the same as the zone of        the old base station BS1, then the user station 102 will        commence execution of an inter-cluster handover as described in        more detail below. Otherwise, it the BSC ID is the same for the        current and target base stations and the zone for both is also        the same, then the user station 102 executes an intra-cluster        handover (see FIGS. 11A-11C). If the observed time slot did not        contain a GENERAL POLL message, then the user station 102        attempts to locate a time slot that has a GENERAL POLL message.        The user station 102 can potentially look at all of the time        slots in which it is not presently communicating and, if        desired, can even skip a transmission on its current time slot        to check the same location time slot on the target base station        BS2 for a GENERAL POLL message.    -   5. If the user station 102 does not yet know the BSC ID of the        target base station BS2, then the user station 102 responds to        the GENERAL POLL message with a GENERAL RESPONSE message and        examines the BSC ID of the GENERAL POLL message. The GENERAL        RESPONSE message sent by the user station 102 includes the user        station's PID,    -   6. After determining that an inter-cluster handover is to be        performed (based upon the BSC ID and/or zone of the old base        station BS1 and that of the target base station BS2), the user        station 102 sends the old base station BS1 an ORIGINATING        HANDOVER REQUEST (CT-OHR) control traffic message. The CT-OHR        message contains the base ID of the preferred new base station        BS2, as determined by the surrounding base station table, as        well as its mobility country code (MCC) and mobility network        code (MNC).    -   7. When the old base station BS1 receives the CT-OHR message,        the old base station BS1 sends an ACKNOWLEDGE (CT-ACK) control        traffic message to the user station 102 to acknowledge the        correct receipt of the CT-OHR message. The old base station BS1        sends an originating Handover Request NOTE to the old base        station controller (BSC1). The Originating Handover Request NOTE        contains the PID of the user station, the old base station ID,        and the MCC and MNC of the target base station BS2. The old base        station BS1 knows the PID of the user station 102 since it was        supplied during the initial slot seizure.    -   8. When the user station 102 receives the CT-ACK message, the        user station 102 and base station 104 resume normal traffic        pending the completion of the circuit switch. If the user        station 102 does not receive a CT-ACK message, the user station        102 assumes that its handover request has not been successful        and it will restart the handover attempt (returning back to Step        4). If it did receive a CT-ACK message, the user station 102        sets an internal user station handover timer with a        predetermined timeout value. The handover is now committed in        the sense that the user station 102 cannot attempt a new        handover until this attempt is completed.    -   9. The old base station controller BSC1 sends a BSSMAP Handover        Required message to the mobile switching center 112 on the SCCP        circuit for the user station 102 (i.e., the SCCP circuit        described by the user station's PID). In a preferred embodiment,        the BSSMAP Handover Required message identifies only a single        cell—the cell serviced by the target base station BS2—in a        preferred cell list.    -   10. The mobile switching center 112 interprets the Handover        Required message and sends a BSSMAP Handover Request message to        the SCCP circuit in the terminating base station controller        (BSC2) that will subsequently be used by the user station 102        upon completion of the handover. The BSSMAP Handover Request        message contains all of the information necessary to maintain        the call in progress, including, e.g., the channel type,        encryption information and priority. In addition, the BSSMAP        Handover Request message contains the base ID of the target base        station BS2.    -   11. The terminating base station controller BSC2 generates a        “handover reference number” and stores the received information        in a small association table for use at a later time. The        information stored in the table is associated with a        concatenation of the handover reference number and the target        base station's base ID. The new base station controller BSC2        then sends a BSSMAP Handover Request ACK message back to the        mobile switching center 112. The BSSMAP Handover Request ACK        message contains the generated handover reference number in its        “level three” information.    -   12. Upon receipt of the BSSMAP Handover Request ACK message, the        mobile switching center 112 sends the old base station        controller BSC1 a BSSMAP Handover Command message on the        original SCCP circuit. The BSSMAP Handover Command message        contains the level three information supplied by the terminating        base station controller BSC2, including the handover reference        number. The handover reference number and the implicit knowledge        of the user station PID (from the SCCP circuit) are all the        identification information needed by the old base station        controller BSC1 to complete the handover.    -   13. After receiving the Circuit Switch Complete NOTE, the old        base station controller BSC1 sends a Circuit Switch Complete        NOTE to the old base station BS1. In place of the connection        number field, this Circuit Switch Complete NOTE contains the        handover reference number from the terminating base station        controller BSC2. The Circuit Switch Complete NOTE also contains        the user station PID that was associated with the SCCP circuit.    -   14. Upon receipt of the Circuit Switch Complete NOTE, the old        base station BS1 sends the user station 102 a CIRCUIT SWITCH        COMPLETE (CT-CSC) control traffic message which contains the        handover reference number. Since the user station 102 has        retained the base ID and frequency of the target base station        BS2, the user station 102 now has all of the information        required to complete the handover. If the old base station BS1        does not receive the Circuit Switch Complete NOTE, an error has        occurred, and the call is torn down.    -   15. Upon receipt of the CT-CSC message, the user station 102        returns an ACKNOWLEDGE (CT-ACK) control traffic message to the        old base station BS1 to acknowledge the correct receipt of the        CT-CSC control traffic message. It the user station 102 does not        receive the CT-CSC message (which contains the handover        reference number needed to complete the handover), the call        cannot be continued on the target base station BS2, and the call        is torn down.    -   16. Upon receipt of the CT-ACK message, the old base station BS1        clears all resources associated with the user station 102 and        makes the channel available for new communication. If the old        base station BS1 does not receive the CT-ACK message, it will        nevertheless clear all resources associated with the user        station 102 and make the channel available for new        communication.    -   17. After sending the CT-ACK message, the user station 102        switches to the frequency of the target base station BS2 and        seizes a channel (i.e., a time slot 302) using the slot seizure        procedure described with respect to FIG. 4. Once the user        station 102 has captured a time slot 302, the user station 102        sends the target base station BS2 a TERMINATING HANDOVER        COMPLETE (CT-THC) control traffic message which contains the        handover reference number and the service request of the user        station 102. If the user station 102 fails to seize a channel on        the target base station BS2, the call is lost. The user station        102 will, in such a case, send a Link Lost message to the user        station application.    -   18. When the target base station BS2 receives the CT-THC        message, it compares the BSC ID of the CT-THC message with the        BSC ID of the base station controller 105 to which it is        connected. This comparison allows the target base station BS2 to        independently determine that an inter-cluster handover is        required. The target base station BS2 responds to the user        station 102 with a BASE ASSIST (BAM) control traffic message to        acknowledge the correct receipt of the CT-THC message. The        target base station BS2 then uses the service request        information element of the CT-THC message to allocate bearer        channels between itself and the terminating base station        controller BSC2, and sends a Terminating Handover Complete NOTE        to the terminating base station controller BSC2. The Terminating        Handover Complete NOTE contains the PID of the user station 102        along with the handover reference number and a description of        the bearer channels assigned to support the user station 102.    -   19. Upon receipt of the CT-RAM message, the user station 102        sends an ACKNOWLEDGE (CT-ACK) control traffic message to the        target base station BS2 to signal correct receipt of the CT-BAM        message.    -   20. The terminating base station BS2 and the user station 102        enter a hold pattern in which they exchange HOLD (CT-HLD)        control traffic messages while awaiting an indication that the        circuit has been switched.    -   21. The terminating base station controller BSC2 uses the        handover reference number and the base ID of the target base        station BS2 to find the associated connection information in the        association table located at the mobile switching center 112. If        the terminating base station controller BSC2 cannot find an        association for the handover reference number and the base ID of        the target base station BS2, then there has been an error and        the call is torn down. Assuming that the proper association is        found, the terminating base station controller BSC2 sends a        Circuit Switch Complete NOTE to the target base station BS2.    -   22. The target base station BS2 responds to the Circuit Switch        Complete NOTE by sending an ACK Circuit Switch Complete NOTE to        the terminating base station controller BSC2.    -   23. When the target base station BS2 receives the Circuit Switch        Complete NOTE, it also sends a CIRCUIT SWITCH COMPLETE (CT-CSC)        control traffic message to the user station 102.    -   24. When the user station 102 receives the CT-CSC message, it        sends an ACKNOWLEDGE (CT-ACK) control traffic message to the        target base station BS2.    -   25. The terminating base station controller BSC2 connects the        bearer channels specified by the target base station BS2 with        the links set up by the mobile switching center 112. The        terminating base station controller BSC2 then sends a Set Cipher        Mode NOTE to the target base station BS2 which contains        ciphering information, if applicable.    -   26. The target base station BS2 uses the ciphering information        to set its ciphering equipment and returns an Acknowledge Cipher        Mode NOTE to the terminating base station controller BSC2. If        the target base station BS2 does not receive the Set Cipher Mode        NOTE, then an error has occurred, and the call is torn down. (If        the Set Cipher Mode NOTE is not received, then the target base        station BS2 does not have the information required to formulate        a CT-SET message to the user station 102, as described in the        following step.)    -   27. The terminating base station controller BSC2 sends a        Handover Detect message to the mobile switching center 112.    -   28. The mobile switching center 112 sends a Handover Complete        message to the terminating base station controller BSC2.    -   29. After receiving the Set Cipher Mode NOTE, the target base        station BS2 sends a SET CIPHER MODE (CT-CIP) control traffic        message to the user station 102.    -   30. When the user station 102 receives the CT-CIP control        traffic message, it sends an ACKNOWLEDGMENT (CT-ACK) control        traffic message to the target base station BS2.    -   31. The mobile switching center 112 sends a Clear Command to the        SCCP circuit of the user station 102 on the old base station        controller BSC1.    -   32. The old base station controller BSC1 clears its resources        that were allocated to the user station 102 and returns a Clear        Complete message to the mobile switching center 112. There is no        need to send any information to the old base station BS1 since        the old base station BS1 cleared all of its resources allotted        to the user station 102 earlier when the old base station        controller BSC1 sent the CT-CSC control traffic message to the        user station 102 in step 14.    -   33. The target base station BS2 clears its base station handover        attempt timer, and the user station 102 likewise clears its user        station handover attempt timer. They enter traffic mode, and the        handover in complete.

Aspects of the invention are directed to facilitating rapid controltraffic within the timing structure of the communication system.Handover, establishing communication, or time slot interchange may becarried out in a rapid manner by utilizing multiple time slots spacedless than one time frame apart. In such a manner, the control traffictakes advantage of unused time slots to avoid having to wait an entiretime frame for each opportunity to exchange messages between the basestation 104 and the user station 102 desiring a transaction. Spareresources are thereby used for the purpose of speeding up controltraffic transactions.

In the preferred embodiment wherein the user station 102 transmits priorto the base station 104 in a time slot 302 (or virtual time slot 618),the slot pointer allows the user station 102 to have knowledge of thenext available time slot 302. Otherwise, the user station 102 may notnecessarily know until a general poll message 401 is received whether ornot a particular time slot is available for communication, and thenwould typically have to wait an entire polling loop before responding tothe general poll message 401.

Knowledge of available time slots 302 is also passed to the user station102 in a specific poll message 402 by use of the OTA map field 726. Asnoted previously, the OTA map field 726 describes the mapping of timeslots relative to a particular user station 102. Thus, for a time frame301 with sixteen time slots 302, the OTA map field 726 in one embodimentcomprises sixteen bits. Each bit may be set to a first value (e.g., “1”)to indicate that the time slot 302 associated with that bit isunavailable, and to a second value (e.g., “0”) to indicate that the timeslot 302 associated with that bit is available for communication.Preferably, the time slot usage is indicated from a standpoint relativeto the current time slot 302 of the user station 302—that is, the firstbit is associated with the immediately following time slot, the secondbit with the next time slot thereafter, the third bit with the next timeslot thereafter, and so on. Alternatively, the time slot usage may beindicated from a standpoint with respect to a fixed reference, such asthe start of the time frame 301, in which case the user station 102needs to have available as information the relative starting point ofthe time frame 301.

FIG. 18A is a timing diagram illustrating rapid control traffic byutilizing multiple time slots within the span of a single time frame. InFIG. 18A, a timing diagram including a plurality of time frames 1401 isshown. A first time frame 1401 a precedes a second time frame 1401 b. Ineach time frame 1401 are a plurality of time slots 1402, numberedconsecutively. Each time frame 1401 has sixteen time slots 1402. Eachtime slot has a user transmission interval 1403 and a base transmissioninterval 1404.

In the first time frame 1401 a, it is assumed that at least three timeslots 1402 (time slots “2”, “8” and “15”) are available. In the secondtime frame 1401 b, it is assumed that at least two time slots 1402 (timeslots “5” and “11”) are available. In time slot “2” of the first timeframe 1401 a, no user station 102 transmission is sent during the usertransmission interval 1403; only a general poll message (e.g., such asgeneral poll message 401 of FIG. 4) is sent by the base station 104during the base transmission interval 1404. The general poll message 401includes a next slot pointer (“NSP”) set to “6”, which indicates thatthe next available slot is six slot positions ahead relative to thecurrent slot; in other words, time slot “8”.

Accordingly, in time slot “8” of the first time frame 1401 a, a userstation 102 desiring to establish communication (either initialcommunication or handover) with the base station 104 transmits a GENERALRESPONSE message (e.g., such as general response message 404 of FIG. 4)during the user transmission interval 1403 of time slot “8”. The basestation 104 receives the GENERAL RESPONSE message, and responds duringthe base transmission interval 1404 of time slot “8” with a SPECIFICPOLL message (e.g., such as specific poll message 402 of FIG. 4). Aspart of the SPECIFIC POLL message, the user station is assigned acorrelative ID (in the present example, the correlative ID is “3”). Thenext slot pointer in the present example is “7”, which means that thenext available slot is seven slot positions ahead relative to thecurrent slot; in other words, time slot “15”.

Accordingly, in time slot “15” of the first time frame 1401 a, the userstation 102 in the present example transmits a control traffic message“THR” (as defined in Table 9-1 above), indicating a “Target HandoverRequest.” In this case, the user station 102 seeks to handover to thebase station 104 from another base station 104. The base station 104responds in the base transmission interval 1404 of time slot “15” with acontrol traffic “ACK” or acknowledge message. The correlative ID of theuser station 102 is sent as part of the acknowledge message, as well asa next slot pointer indicating that the next available slot is six Slotpositions ahead relative to the current sloe; in other words, time slot“5” of the next time frame 1401 b.

Accordingly, in time slot “5” of the second time frame 1401 b, the userstation 102 in the present example transmits a control trafficacknowledge (CT-ACK) message or, alternatively, a control traffic HOLD(CT-HLD) message, as shown in the message flow diagram of FIG. 11. Thebase station 104 then has several options in response. In oneembodiment, the base station 104 may respond with a traffic message inthe base transmission interval 1404 of time slot “5”, provided that thecall has been connected from the base station controller 105.Alternatively, the user station 102 can monitor each time slot 1402until it sees its correlative ID, and then respond thereafter inaccordance with the message directed to it. As another alternative, thebase station 104 may respond in the base transmission interval 1404 witha control traffic message assigning a new time slot 1402 to the userstation 102.

In a preferred embodiment, the user station 102 continues to communicatein the assigned time slot 1402 (i.e., time slot “5”) of each time frameuntil the call is connected and completed, or is otherwise dropped.Until communication is fully established, the base station 104 maytransmit a GENERAL POLL message in the base transmission interval 1404of time slot “5” indicating the next available time slot 1402 for otheruser stations 102 desiring to establish communication.

In one aspect, FIG. 18A illustrates a method of establishingcommunication between a user station 102 and a particular base station104 by exchanging control traffic messages separated by a time durationless than a time frame 1401. In the example shown in FIG. 18A, messagesare exchanged between the user station 102 and the base station 104 inthree time slots 1402 of a first time frame 1401 a, and in two timeslots 1402 of a second time frame 1401 b. This technique can providesubstantial reductions in the amount of time needed to establishcommunication between a user station 102 and a particular base station104, or to handoff communication to a new base station.

Besides being useful for establishing communication (either initialcommunication or handover), the same method may be used to rapidlyexchange control messages between a user station 102 and a base station104, where such rapid exchange is necessary. The rapidity of conductingthe control traffic may be particularly useful, for example, in thesupport of “911” emergency calls or other time-critical situations.

In a particular embodiment, one or more time slots 1402 are reserved for“911” emergency calls, and are not used for non-emergency bearertraffic, For example, four time slots 1402 may be held in reserve. Thesereserved time slots 1402 may also be used to conduct the rapid controltraffic operations described in the FIG. 18A example. Preferably, atleast one time slot 1402 is not used for anything other than receiving apossible “911” emergency call. When a “911” emergency call is received,it may pre-empt other control traffic, and the reserved time slots 1402may be used to conduct a rapid establishment of communication for the“911” call.

The correlative ID assigned to the user station 102 as part of theSPECIFIC POLL message may be used to recover from situations in whichsubsequent messages are received in error due to interference orcorrelation errors. FIG. 18B is a diagram illustrating the rapid controltraffic techniques of FIG. 18A, but wherein one of the messages to theuser station is received in error. In FIG. 18B, similar to FIG. 18A, atiming diagram including a plurality of time frames 1411 is shown. Afirst time frame 1411 a precedes a second time frame 1411 b. Each timeframe 1411 has a plurality (e.g., sixteen) of time slots 1412, numberedconsecutively. As in FIG. 18A, each time slot has a user transmissioninterval 1413 and a base transmission interval 1414.

In FIG. 18B, the same control traffic transactions are carried out intime slots “2” and “8” of the first time frame 1411 a as in FIG. 18A.However, in time slot “15” of the first time frame 1411 a, the basemessage sent in the base transmission interval 1414 is received inerror. As a result, the user station 102 may not know when to expect thenext communication from the base station 104, as the next slot pointerhas been lost. Accordingly, the user station 102 monitors the basetransmission interval 1414 of each time slot 1412 until it recognizesits correlative ID (which was assigned to it as part of the specificpoll message). In the present example, the user station 102 recognizesits correlative ID in time slot “5” of the second time frame 1411 b, andtherefore identifies the message as one intended for it. The userstation 102 also reads the next slot pointer (in this case having avalue of “6”), and therefore responds six time slots 1412 later with anappropriate user message. After recovering from the erroneous receptionin this manner, the exchange between the user station 102 and the basestation 104 may proceed as described with respect to the remaining stepsshown in FIG. 18B.

Thus, loss of the slot pointer does not necessarily prevent theestablishment of communication (or the conducting of other fast controltraffic operations). Recovery from errors is possible by searching forthe correlative ID once communication has been temporarily disrupted byan error in receiving a message from the base station 104.

while the principles of rapid traffic control have been described incertain aspects of FIGS. 18A and 18B with respect to the FIG. 3 timingstructure, the same principles are applicable to the FIG. 6 timingstructure utilizing virtual time slots. The principles are alsoapplicable to hybrid systems using frequency duplex techniques (such asFDD or FDMA) in addition to TDMA/TDD.

FIG. 20 is a block diagram of an exemplary transmitter and receiver in aspread spectrum communication system as may be employed for spreadingand despreading signals in a communication system in accordance with oneor more embodiments of the present invention. In FIG. 20, aspread-spectrum transmitter 2010 a aerial input register 2021, a symboltable 2022, a modulator 2025, a phase selector 2026 and a transmittingantenna 2027 for transmitting a spread-spectrum signal. Aspread-spectrum receiver 2050 comprises a receiver antenna 2051, a downconverter 2052, a bank of spread spectrum demodulators 2056, a best-of-Mdetector 2057, and an output data signal 2059.

In operation, a serial data stream 2012 is received by the transmitter2010 and clocked by a data clock 2013 into the serial input register2021. When N bits have been clocked into the serial input register 2021,one of M spread spectrum codes (or “symbol codes”) is selected from thesymbol table 2022. For example, five bits of the serial data stream 2012clocked into the serial input register 2021 may be used to select one of32 possible symbol codes stored in the symbol table 2022. The selectedsymbol code is output from the symbol table 2022 and used by themodulator 2025 to generate a spread spectrum signal. Another data bit(or possibly multiple data bits, if desired) of the data stream 2012,exclusive from those used to select the symbol code, is input to thephase selector 2026, which determines the phase of the symbol codeselected from the symbol table 2022. For example, the phase selector mayuse a single bit (called a “Phase control bit” of the data stream 2012to determine the phase of the symbol code; if this phase control bit hasa first value (e.g., a “0”), then the symbol code is ID transmitted withno phase inversion, while if the phase control bit has a second value(e.g., a “1”), then the symbol code is transmitted with a phaseinversion of 180 degrees. If two phase control bits are used, then fourpossible phases could be selected, and so on for additional phasecontrol bits.

The modulator 2025 transmits the selected symbol code using the phaseindicated by the phase selector 2026. The modulator 2025 may transmitusing continuous phase modulation, or a similar technique, so as tominimize spectral splatter. In the transmission process, the modulator2025 preferably modulates the selected symbol code with a carrier signalat a predetermined carrier frequency. Exemplary spread spectrummodulators are described in, for example, U.S. Pat. Nos. 5,548,253 and5,659,574, both of which is assigned to the assignee of the presentinvention, and both of which are hereby incorporated by reference as ifset forth fully herein.

At the spread-spectrum receiver 2050, the transmitted spread spectrumsignal is received at the receiver antenna 2051 and down-converted tobaseband by the down converter 2052. The baseband signal is then fed toa bank of spread spectrum demodulators 2056, each of which is configuredto recognize one of the M possible symbol codes, and each of whichoutputs a correlation signal indicating a degree of match with itsrespective symbol code. The best-of-M detector 2057 receives thecorrelation signal from each of the spread spectrum demodulators 2056,and determines which of the M symbol codes has been received based onthe relative strengths of the correlation signals. The best-of-Mdetector 2057 generates an output data signal 2059 based upon thereceived symbol codes. The phase of the received symbol code can also bedetected, and further information received by differential phasedecoding.

Exemplary correlators suitable for use with certain embodiments of thepresent invention are described in, among other places, U.S. Pat. Nos.5,022,047 and 5,016,25, both of which are assigned to the assignee ofthe present invention, and both of which are incorporated by referenceas if fully set forth herein. A preferred method of correlation isdescribed in U.S. Pat. No. 5,659,574 issued Aug. 5, 1997, assigned tothe assignee of the present invention, and hereby incorporated byreference as if set forth fully herein. In particular, a multi-bitcorrelation technique as described in U.S. Pat. No. 5,659,574 representsa presently preferred manner of correlating a spread spectrum signal.U.S. Pat. No. 5,659,574 also sets forth a presently preferred techniqueof differential phase encoding and decoding usable in conjunction withthe present invention.

Spread spectrum communication techniques are further described in, e.g.,Robert C. Dixon, Spread Spectrum System with commercial Applications(John Wiley & Sons, 3d ed. 1994), hereby incorporated by reference as ifset forth fully herein. A large variety of spread spectrum systems havebeen proposed in the industry, and the particular details of the spreadspectrum system set forth above are in no way meant to be limiting tothe scope of the invention. Moreover, while spread spectrumcommunication techniques are utilized in a preferred embodiment of theinvention, many embodiments of the invention are operable without usingspread spectrum.

Several further variations, modifications and enhancements of theinvention will now be described. User stations 102 in one embodiment maycomprise mobile handsets capable of multi-band and/or multi-modeoperation. The user stations 102 may be multi-mode in that they may becapable of both spread spectrum (i.e., wideband) communication and alsonarrowband communication. The user stations 102 may be multi-band in thesense that they may be set to operate on a plurality of differentfrequencies, such as frequencies in either the licensed or unlicensedPCS bands. The user stations 102 may operate in one mode (e.g.,wideband) over a first frequency band, and another mode (e.g.,narrowband) over a second frequency band.

As an example, a user Station 102 may be set to operate on a pluralityof frequencies between 1850 and 1990 MHz, with the frequencies separatedin 625 kHz steps. Each user station 102 may be equipped with a frequencysynthesizer that may be programmed to allow reception and/ortransmission on any one of the plurality of frequencies. If the userstation 102 operates solely in a licensed PCS band (e.g., 1850 MHz toMHz), the programmable frequency steps may be in 5 MHz increments, inwhich case the first channel may be centered at 1852.5 MHz, the next at1857.5 MHz, and so on. If operating in the isochronous band between 1920and 1930 MHz, the first channel may be centered at 1920.625 MHz, and thechannel spacing may be 1.25 MHz across the remainder of the isochronousband. The user stations 102 may or may not be configured to operate inthe 1910 to 1920 MHz band, which at present is set apart in the UnitedStates for asynchronous unlicensed devises.

Further information regarding dual-mode and/or dual-band communicationis set forth in U.S. patent application Ser. No. 08/483,514 filed onJun. 7, 1995, hereby incorporated by reference as if set forth fullyherein.

In one embodiment, a communication protocol provides channel informationto a base station to select an antenna for communication with a userstation 102. Further, the protocol provides for output power adjustmentin a user station 102 and a base station 104. A preferred poweradjustment command from the base station 104 to the user station 102 maybe encoded according to Table 8-2 appearing earlier herein. Althoughpreferred values are provided in Table 8-2, the number of power controlcommand steps and the differential in power adjustment between steps mayvary depending upon the particular application and the systemspecifications. Further information regarding antenna diversity andpower adjustment technique may be found in copending U.S. patentapplication Ser. No. 08/826,773 filed on Apr. 7, 1997, herebyincorporated by reference as if set forth fully herein.

The present invention has been set forth in the form of its preferredembodiments. It is nevertheless understood that modifications andvariations of the disclosed techniques for carrying out fast controltraffic, and for establishing and maintaining spread spectrumcommunication, may be apparent to those skilled in the art withoutdeparting from the scope and spirit of the present invention. Moreover,such modifications are considered to be within the purview of theappended claims.

1. A method comprising transmitting a plurality of base-to-user messagesfrom a base station to a user station within time slots of a single timeframe, each time frame having a plurality of duplex time slots, and eachtime slot having a base transmission interval and a user transmissioninterval, the base-to-user messages alternating with one or moreuser-to-base messages, each of the base-to-user messages having aninformation element indicating a location of a subsequent time slotavailable for communication; wherein the base-to-user messages compriseat least one general poll message broadcast to user stations and atleast one specific poll message directed to a specific user station, andwherein the user-to-base messages comprises at least one generalresponse message transmitted by a user station in response to the atleast one general poll message.
 2. The method of claim 1, wherein theplurality of base-to-user and user-to-base messages comprise controltraffic messages, and wherein transmitting the plurality of base-to-usermessages comprises completing a control traffic transaction.
 3. Themethod of claim 1, further comprising completing a handshake transactionbetween the base station and the user station so as to establish acommunication channel and exchanging bearer traffic messages between theuser station and the base station over the established communicationchannel.
 4. The method of claim 1, wherein transmitting the plurality ofbase-to-user messages comprises spread spectrum encoding thebase-to-user messages.
 5. The method of claim 1, wherein theuser-to-base messages are each in the respective time slot indicated bythe information element of the preceding base-to-user message from thebase station.
 6. The method of claim 1, wherein the duplex time slotscomprise virtual time slots.
 7. The method of claim 1, wherein theinformation element comprises a numerical value indicating a relativenumber of time slots until the subsequent time slot.
 8. The method ofclaim 1, wherein the information element indicates a location of asubsequent time slot available to a user station for communication.
 9. Amethod for communicating between a base station and a user station in awireless communication network using a message structure, the methodcomprising: transmitting from the base station to a message recipient ata user station a data segment within a time slot of a time frame whereineach time frame comprises a plurality of duplex time slots, and eachtime slot comprises a base transmission interval and a user transmissioninterval; and transmitting a header segment within the time slot andadjoining the data segment, the header segment including a next slotpointer to indicate a subsequent available time slot for communicationby the message recipient at the user station, the subsequent time slotbeing designated without regard to whether the time slot is in the sametime frame or not.
 10. The method of claim 9, wherein the next slotpointer comprises a numerical value indicating a relative number of timeslots until the subsequent time slot is available for communication. 11.The method of claim 9, wherein the next slot pointer comprises anumerical value indicating an absolute position of the subsequent timeslot available for communication relative to a starting point of a timeframe.
 12. The method of claim 9, wherein the duplex time slots arevirtual time slots.
 13. A method comprising: transmitting a firstplurality of control traffic messages from a user station to a basestation in a user transmission interval of a first plurality of timeslots, at least two of the first plurality of time slots being within asingle time frame; receiving a second plurality of control trafficmessages from the base station at the user station in a basetransmission interval of a second plurality of time slots, at least oneof the second plurality of control traffic messages comprising a nextslot pointer indicating to the user station an available time slot fortransmitting one of the first plurality of control traffic messages, atleast two of the second plurality of time slots being within the singletime frame, wherein the time slots of the first plurality of time slotsand of the second plurality of time slots are duplex time slots.
 14. Themethod of claim 13 wherein transmitting the plurality of control trafficmessages comprises spread spectrum encoding the first plurality ofcontrol traffic message, and wherein transmitting the second pluralityof control traffic messages comprises spread spectrum encoding thesecond plurality of control traffic messages.
 15. The method of clam 13wherein the first plurality of control traffic messages and the secondplurality of control traffic messages are transmitted over the samefrequency band.
 16. The method of claim 13, further comprising:completing a handshake transaction between the base station and a userstation using the first and second plurality of control traffic messagesso as to establish a communication channel; and exchanging bearertraffic messages between the user station and the base station over theestablished communication channel.
 17. The method of claim 13, furthercomprising transmitting a second plurality of user-to-base messages fromthe user station to the base station, each of the plurality ofuser-to-base messages transmitted in the respective time slot indicatedby the information element of the preceding base-to-user control trafficmessage from the base station.
 18. The method of claim 13, wherein theplurality of time slots are virtual time slots.
 19. The method of claim13, wherein the plurality of time slots are virtual time slots, suchthat the user transmission interval of a time slot is not adjacent intime to the base transmission interval of the time slot.
 20. The methodof claim 13, wherein at least two of the base station control traffictransmissions occurring within the timespan of a single time frame,separated by at least one of the user station control traffictransmissions, the base station control traffic transmissions eachcontaining an information element pointing to a previously unassignedtime slot, and the user station control traffic transmissions are eachtransmitted in the time slot identified by the information element inthe immediately previous base station control traffic transmission. 21.A multiple-user communication system, comprising: a base station togenerate a series of time frames, each of the time frames comprising aplurality of time slots each having a base transmission interval and auser transmission interval; wherein the base station transmits controltraffic messages during selected ones of the time slots, each controltraffic message comprising a next slot pointer identifying a subsequenttime slot available to a user station for communication; wherein thebase station receives in the time slot identified by the next slotpointer of that control traffic message from a user station respondingto one of the control traffic messages; and wherein the base stationexchange at least three control traffic messages in alternatingsuccession with the responding user station and within the timespan of asingle time frame.
 22. The multiple-user system of claim 21 wherein basestation transmissions are transmitted in a spread spectrum format. 23.The multiple-user system of claim 21 wherein each of the control trafficmessages transmitted by the base station comprise a base header segment,and wherein the next slot point is contained within the base headersegment.
 24. The multiple-user communication system of claim 21, whereinthe plurality of time slots are virtual time slots.
 25. A multiple-userwireless communication system, comprising: a series of time frames eachdivided into a plurality of time slots collectively comprising aplurality of user transmission intervals and a plurality of basetransmission intervals, each of the time slots comprising a usertransmission interval followed by a base transmission interval such thatthe user transmission intervals alternate with the base transmissionintervals within each time frame; and a user station; wherein the userstation receives, over a designated frequency band, one or morebase-to-user control traffic messages, each of the base-to-user controltraffic messages transmitted in the base transmission interval of one ofthe time slots, at least one of the base-to-user control trafficmessages comprising an information element indicating a location of asubsequent time slot available for communication; and wherein the userstation transmits in response to the base-to-user control trafficmessages, over the designated frequency band, a user-to-base controltraffic message to the base station, in the user transmission intervalof one of the time slots indicated by the information element of thepreceding base-to-user control traffic message from the base station;wherein the base-to-user control traffic messages comprise at least onegeneral poll message broadcast to user stations and at least onespecific poll message directed to the user station, and wherein theuser-to-base control traffic message comprises at least one generalresponse message transmitted by the user station in response to the atleast one general poll message.
 26. The multiple user wirelesscommunication system of claim 25, wherein the user station communicateswith the base station in time division duplex within selected time slotsthrough the exchange of data traffic messages.
 27. A method comprising:transmitting a first message from a base station in a wirelesscommunication network to a user station, the first message comprising adata segment and an information element indicating a location of asubsequent time slot available for communication; and receiving a secondmessage at the base station from the user station in the indicated timeslot; wherein the time slot comprises a duplex time slot, and whereinthe duplex time slot comprises a virtual time slot comprising a forwardlink transmission interval and a corresponding reverse link transmissioninterval that are non-adjacent in time.
 28. The method of claim 27,wherein the first message comprises a base-to-user message and thesecond message comprises a user-to-base message.
 29. The method of claim27, wherein the first and second messages are transmitted over the samefrequency band.
 30. The method of claim 27, wherein the time framecomprises a plurality of duplex time slots.